Light-emitting element, light-emitting device, display device, electronic device, and lighting device

ABSTRACT

A light-emitting element which uses a plurality of kinds of light-emitting dopants emitting light in a balanced manner and has high emission efficiency is provided. Further, a light-emitting device, a display device, an electronic device, and a lighting device each having reduced power consumption by using the above light-emitting element are provided. A light-emitting element which includes a plurality of light-emitting layers including different phosphorescent materials is provided. In the light-emitting element, the light-emitting layer which includes a light-emitting material emitting light with a long wavelength includes two kinds of carrier-transport compounds having properties of transporting carriers with different polarities. Further, in the light-emitting element, the triplet excitation energy of a host material included in the light-emitting layer emitting light with a short wavelength is higher than the triplet excitation energy of at least one of the carrier-transport compounds.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light-emitting element, a displaydevice, a light-emitting device, an electronic device, and a lightingdevice each of which includes an organic compound as a light-emittingsubstance.

2. Description of the Related Art

In recent years, research and development have been extensivelyconducted on light-emitting elements using electroluminescence (EL). Ina basic structure of such a light-emitting element, a layer containing alight-emitting substance (an EL layer) is interposed between a pair ofelectrodes. By applying voltage to this element, light emission from thelight-emitting substance can be obtained.

Since such a light-emitting element is of self-light-emitting type, thelight-emitting element has advantages over a liquid crystal display inthat visibility of pixels is high, a backlight is not required, and soon and is therefore thought to be suitable as flat panel displayelements. In addition, it is also a great advantage that a displayincluding such a light-emitting element can be manufactured as a thinand lightweight display. Furthermore, very high speed response is alsoone of the features of such an element.

Since a light-emitting layer of such a light-emitting element can beformed in the form of a film, planar light emission can be achieved.Therefore, large-area elements can be easily formed. This feature isdifficult to obtain with point light sources typified by incandescentlamps and LEDs or linear light sources typified by fluorescent lamps.Thus, light-emitting elements also have great potential as planar lightsources applicable to lighting devices and the like.

In the case of an organic EL element in which an EL layer containing anorganic compound as the light-emitting substance is provided between apair of electrodes, application of a voltage between the pair ofelectrodes causes injection of electrons from the cathode and holes fromthe anode into the EL layer having a light-emitting property, and thus acurrent flows. By recombination of the injected electrons and holes, theorganic compound having a light-emitting property is excited andprovides light emission from the excited state.

The excited state of an organic compound can be a singlet excited stateor a triplet excited state, and light emission from the singlet excitedstate (S*) is referred to as fluorescence, and light emission from thetriplet excited state (T*) is referred to as phosphorescence. Thestatistical generation ratio of the excited states in the light-emittingelement is considered to be S*:T*=1:3.

In a compound that emits light from the singlet excited state(hereinafter, referred to as a fluorescent compound), at roomtemperature, generally light emission from the triplet excited state(phosphorescence) is not observed while only light emission from thesinglet excited state (fluorescence) is observed. Therefore, theinternal quantum efficiency (the ratio of generated photons to injectedcarriers) of a light-emitting element using a fluorescent compound isassumed to have a theoretical limit of 25% based on the ratio of S* toT* which is 1:3.

In contrast, in a compound that emits light from the triplet excitedstate (hereinafter, referred to as a phosphorescent compound), lightemission from the triplet excited state (phosphorescence) is observed.Further, in a phosphorescent compound, since intersystem crossing (i.e.,transfer from a singlet excited state to a triplet excited state) easilyoccurs, the internal quantum efficiency can be increased to 100% intheory. That is, higher emission efficiency can be achieved than using afluorescent compound. For this reason, light-emitting elements usingphosphorescent compounds are now under active development in order toobtain highly efficient light-emitting elements.

A white light-emitting element disclosed in Patent Document 1 includes alight-emitting region containing a plurality of kinds of light-emittingdopants which emit phosphorescence.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Translation of PCT International    Application No. 2004-522276

SUMMARY OF THE INVENTION

Although an internal quantum efficiency of 100% in a phosphorescentcompound is theoretically possible, such high efficiency can be hardlyachieved without optimization of an element structure or a combinationwith another material. Especially in a light-emitting element whichincludes a plurality of kinds of phosphorescent compounds havingdifferent bands (different emission colors) as light-emitting dopants,it is difficult to obtain highly efficient light emission without notonly considering energy transfer but also optimizing the efficiency ofthe energy transfer. In fact, in Patent Document 1, even when all thelight-emitting dopants of a light-emitting element are phosphorescentcompounds, the external quantum efficiency is approximately 3% to 4%. Itis thus presumed that even when light extraction efficiency is takeninto account, the internal quantum efficiency is 20% or lower, which islow for a phosphorescent light-emitting element.

In a multicolor light-emitting element using dopants exhibitingdifferent emission colors, beside improvement of emission efficiency, itis also necessary to attain a good balance between light emissions bythe dopants which exhibit different emission colors. It is not easy tokeep a balance between light emissions by the dopants and to achievehigh emission efficiency at the same time.

In view of the above, an object of one embodiment of the presentinvention is to provide a light-emitting element which uses a pluralityof kinds of light-emitting dopants emitting light in a balanced mannerand has high emission efficiency. Another object of one embodiment ofthe present invention is to provide a light-emitting device, a displaydevice, an electronic device, and a lighting device each having reducedpower consumption by using the above light-emitting element.

It is only necessary that at least one of the above-described objects beachieved in the present invention.

According to the present invention, a light-emitting element whichincludes a plurality of light-emitting layers including differentphosphorescent materials is provided. The light-emitting layer of theplurality of light-emitting layers which includes a light-emittingmaterial emitting light with a long wavelength includes two kinds ofcarrier-transport compounds having properties of transporting carrierswith different polarities. Further, the light-emitting layer of theplurality of light-emitting layers which includes a light-emittingmaterial emitting light with a short wavelength includes a hostmaterial, and the triplet excitation energy of the host material ishigher than the triplet excitation energy of at least one of thecarrier-transport compounds. By such a combination of a host materialand a carrier-transport compound with which energy can be transferredefficiently between hosts, a light-emitting element of one embodiment ofthe present invention can be provided.

That is, one embodiment of the present invention is a light-emittingelement including: a first light-emitting layer including a firstphosphorescent compound and a first host material, between an anode anda cathode; and a second light-emitting layer including a secondphosphorescent compound, a first electron-transport compound, and afirst hole-transport compound, between the anode and the cathode. Anemission wavelength of the second phosphorescent compound is longer thanan emission wavelength of the first phosphorescent compound. A tripletexcitation energy of the first host material is higher than or equal toa triplet excitation energy of the first electron-transport compound orthe first hole-transport compound. The first light-emitting layer andthe second light-emitting layer are in contact with each other.

Another embodiment of the present invention is the light-emittingelement in which the first electron-transport compound and the firsthole-transport compound form an exciplex.

Another embodiment of the present invention is the light-emittingelement in which the first host material is an electron-transportcompound, and in which the first light-emitting layer is closer to thecathode than the second light-emitting layer.

Another embodiment of the present invention is the light-emittingelement in which the first host material is a hole-transport compound,and in which the first light-emitting layer is closer to the anode thanthe second light-emitting layer.

Another embodiment of the present invention is the light-emittingelement in which the first light-emitting layer further includes asecond host material, in which the second host material is ahole-transport compound, and in which a triplet excitation energy of thesecond host material is higher than or equal to the triplet excitationenergy of the first electron-transport compound or the firsthole-transport compound.

Another embodiment of the present invention is the light-emittingelement in which the first light-emitting layer further includes asecond host material, in which the second host material is anelectron-transport compound, and in which a triplet excitation energy ofthe second host material is higher than or equal to the tripletexcitation energy of the first electron-transport compound or the firsthole-transport compound.

Another embodiment of the present invention is the light-emittingelement in which the first host material and the second host materialform an exciplex.

Another embodiment of the present invention is a light-emitting elementincluding: a first light-emitting layer including a first phosphorescentcompound and a first host material, between an anode and a cathode; asecond light-emitting layer including a second phosphorescent compound,a first electron-transport compound, and a first hole-transportcompound, between the anode and the cathode; and a third light-emittinglayer including a third phosphorescent compound, a secondelectron-transport compound, and a second hole-transport compound,between the anode and the cathode. An emission wavelength of the secondphosphorescent compound is longer than an emission wavelength of thefirst phosphorescent compound. An emission wavelength of the thirdphosphorescent compound is longer than the emission wavelength of thesecond phosphorescent compound. A triplet excitation energy of the firsthost material is higher than or equal to a triplet excitation energy ofthe first electron-transport compound or the first hole-transportcompound. The second light-emitting layer is in contact with the firstlight-emitting layer, and the third light-emitting layer is in contactwith the second light-emitting layer.

Another embodiment of the present invention is a light-emitting elementincluding: a first light-emitting layer including a first phosphorescentcompound and a first host material, between an anode and a cathode; asecond light-emitting layer including a second phosphorescent compound,a first electron-transport compound, and a first hole-transportcompound, between the anode and the cathode; and a third light-emittinglayer including a third phosphorescent compound, a secondelectron-transport compound, and a second hole-transport compound,between the anode and the cathode. An emission wavelength of the secondphosphorescent compound is longer than an emission wavelength of thefirst phosphorescent compound. An emission wavelength of the thirdphosphorescent compound is longer than the emission wavelength of thesecond phosphorescent compound. A triplet excitation energy of the firsthost material is higher than or equal to a triplet excitation energy ofthe first electron-transport compound or the first hole-transportcompound. The triplet excitation energies of the firstelectron-transport compound and the first hole-transport compound arehigher than a triplet excitation energy of the second electron-transportcompound or the second hole-transport compound. The secondlight-emitting layer is in contact with the first light-emitting layer,and the third light-emitting layer is in contact with the secondlight-emitting layer.

Another embodiment of the present invention is the light-emittingelement in which the first electron-transport compound and the firsthole-transport compound form an exciplex, and in which the secondelectron-transport compound and the second hole-transport compound forman exciplex.

Another embodiment of the present invention is the light-emittingelement in which the first host material is an electron-transportcompound, and in which the first light-emitting layer is closer to thecathode than the second light-emitting layer.

Another embodiment of the present invention is the light-emittingelement in which the first host material is a hole-transport compound,and in which the first light-emitting layer is closer to the anode thanthe second light-emitting layer.

Another embodiment of the present invention is the light-emittingelement in which the first light-emitting layer further includes asecond host material, in which the second host material is ahole-transport compound, and in which a triplet excitation energy of thesecond host material is higher than or equal to the triplet excitationenergy of the first electron-transport compound or the firsthole-transport compound.

Another embodiment of the present invention is the light-emittingelement in which the first light-emitting layer further includes asecond host material, in which the second host material is anelectron-transport compound, and in which a triplet excitation energy ofthe second host material is higher than or equal to the tripletexcitation energy of the first electron-transport compound or the firsthole-transport compound.

Another embodiment of the present invention is the light-emittingelement in which the first host material and the second host materialform an exciplex.

Another embodiment of the present invention is the light-emittingelement in which the first electron-transport compound and the secondelectron-transport compound are the same material.

Another embodiment of the present invention is the light-emittingelement in which the first hole-transport compound and the secondhole-transport compound are the same material.

Another embodiment of the present invention is the light-emittingelement in which the first electron-transport compound and the secondelectron-transport compound are the same material, and in which thefirst hole-transport compound and the second hole-transport compound arethe same material.

Another embodiment of the present invention is the light-emittingelement in which a thickness of the second light-emitting layer isgreater than or equal to 2 nm and less than or equal to 20 nm.

Another embodiment of the present invention is the light-emittingelement in which the thickness of the second light-emitting layer isgreater than or equal to 5 nm and less than or equal to 10 nm.

Another embodiment of the present invention is a light-emitting device,a light-emitting display device, an electronic device, and a lightingdevice each including the light-emitting element.

Note that the light-emitting device in this specification includes, inits category, an image display device with a light-emitting element.Further, the category of the light-emitting device in this specificationincludes a module in which a light-emitting device is provided with aconnector such as an anisotropic conductive film or a TCP (tape carrierpackage); a module in which the end of the TCP is provided with aprinted wiring board; and a module in which an IC (integrated circuit)is directly mounted on a light-emitting device by a COG (chip on glass)method. Furthermore, the category includes light-emitting devices thatare used in lighting equipment or the like.

One embodiment of the present invention can provide a light-emittingelement having high emission efficiency. By using the light-emittingelement, another embodiment of the present invention can provide any ofa light-emitting device, a light-emitting display device, an electronicdevice, and a lighting device each having reduced power consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, 1C, 1D, and 1E are schematic diagrams of light-emittingelements.

FIG. 2 is a schematic diagram of a light-emitting element.

FIGS. 3A and 3B are schematic diagrams of an active matrixlight-emitting device.

FIGS. 4A and 4B are schematic diagrams of a passive matrixlight-emitting device.

FIGS. 5A and 5B are each a schematic diagram of an active matrixlight-emitting device.

FIG. 6 is a schematic diagram of an active matrix light-emitting device.

FIGS. 7A and 7B are schematic diagrams of a lighting device.

FIGS. 8A, 8B1, 8B2, 8C, and 8D illustrate electronic devices.

FIG. 9 illustrates an electronic device.

FIG. 10 illustrates a lighting device.

FIG. 11 illustrates a lighting device.

FIG. 12 illustrates in-vehicle display devices and lighting devices.

FIGS. 13A to 13C illustrate an electronic device.

FIG. 14 shows an emission spectrum of a light-emitting element 1.

FIG. 15 shows luminance-current efficiency characteristics of thelight-emitting element 1.

FIG. 16 shows luminance-external quantum efficiency characteristics ofthe light-emitting element 1.

FIG. 17 shows voltage-luminance characteristics of the light-emittingelement 1.

FIG. 18 shows luminance-power efficiency characteristics of thelight-emitting element 1.

FIG. 19 shows emission spectra of a light-emitting element 2 and alight-emitting element 3.

FIG. 20 shows luminance-current efficiency characteristics of thelight-emitting element 2 and the light-emitting element 3.

FIG. 21 shows luminance-external quantum efficiency characteristics ofthe light-emitting element 2 and the light-emitting element 3.

FIG. 22 shows voltage-luminance characteristics of the light-emittingelement 2 and the light-emitting element 3.

FIG. 23 shows luminance-power efficiency characteristics of thelight-emitting element 2 and the light-emitting element 3.

FIG. 24 shows an emission spectrum of a light-emitting element 4.

FIG. 25 shows luminance-current efficiency characteristics of thelight-emitting element 4.

FIG. 26 shows luminance-external quantum efficiency characteristics ofthe light-emitting element 4.

FIG. 27 shows voltage-luminance characteristics of the light-emittingelement 4.

FIG. 28 shows luminance-power efficiency characteristics of thelight-emitting element 4.

FIG. 29 shows time dependence of normalized luminance of thelight-emitting element 4.

FIG. 30 shows a phosphorescent spectrum of 35DCzPPy.

FIG. 31 shows a phosphorescent spectrum of PCCP.

FIG. 32 shows a phosphorescent spectrum of 2mDBTPDBq-II.

FIG. 33 shows a phosphorescent spectrum of PCBA1BP.

FIG. 34 shows a phosphorescent spectrum of 2mDBTBPDBq-II.

FIG. 35 shows a phosphorescent spectrum of PCBNBB.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments and examples of the present invention will bedescribed in detail with reference to the accompanying drawings. Notethat the present invention is not limited to the following description,and it will be easily understood by those skilled in the art thatvarious changes and modifications can be made without departing from thespirit and scope of the present invention. Therefore, the presentinvention should not be construed as being limited to the description inthe following embodiments and examples.

Embodiment 1

First, the operation principle of a light-emitting element of oneembodiment of the present invention will be described. The point of thepresent invention is that a first phosphorescent compound and a secondphosphorescent compound emitting light whose wavelength is longer thanthat of light emitted from the first phosphorescent compound are usedand both of the first and second phosphorescent compounds are made toemit light efficiently, whereby a multicolor light-emitting element withhigh efficiency is obtained.

As a general method for obtaining a multicolor light-emitting elementincluding a phosphorescent compound, a method can be given in which aplurality of kinds of phosphorescent compounds having different emissioncolors are dispersed in some host material in an appropriate ratio.However, in such a method, the phosphorescent compound which emits lightwith the longest wavelength readily emits light, so that it is extremelydifficult to design and control an element structure (especially theconcentrations of the phosphorescent compounds in the host material) forobtaining polychromatic light.

As another technique for obtaining a multicolor light-emitting element,what is called a tandem structure, in which light-emitting elementshaving different emission colors are stacked in series, can be given.For example, a blue light-emitting element, a green light-emittingelement, and a red light-emitting element are stacked in series and madeto emit light at the same time, whereby polychromatic light (in thiscase, white light) can be easily obtained. The element structure can berelatively easily designed and controlled because the bluelight-emitting element, the green light-emitting element, and the redlight-emitting element can be independently optimized. However, thestacking of three elements is accompanied by an increase in the numberof layers and makes the fabrication complicated. In addition, when aproblem occurs in electrical contact at connection portions between theelements (what is called intermediate layers), an increase in drivevoltage, i.e., power loss might be caused.

In contrast, in a light-emitting element of one embodiment of thepresent invention, a first light-emitting layer and a secondlight-emitting layer are stacked between a pair of electrodes. The firstlight-emitting layer includes a first phosphorescent compound and afirst host material. The second light-emitting layer includes a secondphosphorescent compound emitting light whose wavelength is longer thanthat of the first phosphorescent compound, a first electron-transportcompound, and a first hole-transport compound. Here, the tripletexcitation energy of the first host material is higher than that of thefirst electron-transport compound or the first hole-transport compound,and the first light-emitting layer and the second light-emitting layerare provided in contact with each other unlike in a tandem structure.

FIG. 1E schematically illustrates the element structure of thelight-emitting element of one embodiment of the present invention. InFIG. 1E, a first electrode 101, a second electrode 102, and an EL layer103 are illustrated. The EL layer 103 includes at least a light-emittinglayer 113 and other layers may be provided as appropriate. In thestructure illustrated in FIG. 1E, a hole-injection layer 111, ahole-transport layer 112, an electron-transport layer 114, and anelectron-injection layer 115 are assumed to be provided. Note that it isassumed that the first electrode 101 functions as an anode and thesecond electrode 102 functions as a cathode.

FIGS. 1A and 1B are each an enlarged view of the light-emitting layer113 in the light-emitting element. In each of FIGS. 1A and 1B, a firstlight-emitting layer 113 a, a second light-emitting layer 113 b, thelight-emitting layer 113 which is a combination of the two layers, afirst phosphorescent compound 113Da, a second phosphorescent compound113Db, a first host material 113Ha1, a first carrier-transport compound113H₁, and a second carrier-transport compound 113H₂ are illustrated.FIG. 1B is a schematic diagram illustrating the case where the firstlight-emitting layer 113 a further includes a second host material113Ha2. Note that the first host material 113Ha1 and the firstcarrier-transport compound 113H₁ may be the same or different from eachother; and the second host material 113Ha2 and the secondcarrier-transport compound 113H₂ may be the same or different from eachother. The first light-emitting layer 113 a may be on the anode side andthe second light-emitting layer 113 b may be on the cathode side, or thefirst light-emitting layer 113 a may be on the cathode side and thesecond light-emitting layer 113 b may be on the anode side. Note thatone of the first host material 113Ha1 and the second host material113Ha2 is an electron-transport compound, and the other of them is ahole-transport compound. Similarly, one of the first carrier-transportcompound 113H₁ and the second carrier-transport compound 113H₂ is anelectron-transport compound, and the other of them is a hole-transportcompound.

The position of a recombination region in the light-emitting layer canbe adjusted with the mixture ratio of the first host material 113Ha1 tothe second host material 113Ha2, the first carrier-transport compound113H₁, and the second carrier-transport compound 113H₂ which areincluded in the light-emitting layers. As described above, one of thefirst host material 113Ha1 and the second host material 113Ha2 is anelectron-transport compound, and the other of them is a hole-transportcompound; and one of the first carrier-transport compound 113H₁ and thesecond carrier-transport compound 113H₂ is an electron-transportcompound, and the other of them is a hole-transport compound. For such areason, changing the mixture ratio thereof can adjust thecarrier-transport property of each light-emitting layer, and accordinglycan easily control the position of the recombination region.

Note that light emission from the first phosphorescent compound 113Da isdifficult to obtain in the case where an exciton is directly generatedin the second light-emitting layer 113 b; therefore, the carrierrecombination region is preferably in the first light-emitting layer 113a or in the vicinity of the interface between the first light-emittinglayer 113 a and the second light-emitting layer 113 b.

In order that the carrier recombination region is in the vicinity of theinterface between the first light-emitting layer 113 a and the secondlight-emitting layer 113 b, the first light-emitting layer 113 a is madeto have a hole-transport property in the case where the firstlight-emitting layer 113 a is on the anode side, or is made to have anelectron-transport property in the case where the first light-emittinglayer 113 a is on the cathode side. Further, the second light-emittinglayer 113 b is made to have an opposite carrier-transport property tothat of the first light-emitting layer 113 a; thus, the recombinationregion can be made in the vicinity of the interface between the firstlight-emitting layer 113 a and the second light-emitting layer 113 b. Inorder that the carrier recombination region is in the firstlight-emitting layer 113 a, the bipolar property of the firstlight-emitting layer 113 a is improved with the above structure as abase.

In the case of the structure illustrated in FIG. 1A, the first hostmaterial 113Ha1 is made to have a hole-transport property in the casewhere the first light-emitting layer 113 a is on the anode side, or ismade to have an electron-transport property in the case where the firstlight-emitting layer 113 a is on the cathode side; the secondlight-emitting layer 113 b is made to have an opposite carrier-transportproperty to that of the first light-emitting layer 113 a by changing themixture ratio of the first carrier-transport compound 113H₁ to thesecond carrier-transport compound 113H₂.

In the case of a combination of the second light-emitting layer 113 band the first light-emitting layer 113 a illustrated in FIG. 1C, thecarrier-transport properties of the first light-emitting layer 113 a andthe second light-emitting layer 113 b can be adjusted by changing themixture ratio of the electron-transport compound to the hole-transportcompound in each of the light-emitting layers.

Note that when the recombination region is in the first light-emittinglayer 113 a or at the interface between the first light-emitting layerand the second light-emitting layer, the intensity of light emissionfrom the second phosphorescent compound 113Db is lower than that oflight emission from the second phosphorescent compound 113Da in somecases. In view of this, in one embodiment of the present invention, acombination of materials is selected such that the triplet excitationenergy of the first host material 113Ha1 is higher than that of thefirst carrier-transport compound 113H₁ and/or that of the secondcarrier-transport compound 113H₂ in the case of the structureillustrated in FIG. 1A. The triplet excitation energy due torecombination of carriers partly moves from the triplet excitation levelof the first host material 113Ha1 to that of the first carrier-transportcompound 113H₁ and/or that of the second carrier-transport compound113H₂; in this manner, the second phosphorescent compound 113Db can emitlight.

In the case where the first light-emitting layer 113 a further includesthe second host material 113Ha2 as illustrated in FIG. 1B, a combinationof materials is selected such that the triplet excitation energy of thesecond host material 113Ha2 is higher than that of the firstcarrier-transport compound 113H₁ and/or that of the secondcarrier-transport compound 113H₂. The triplet excitation energy due torecombination of carriers partly moves from the triplet excitation levelof the second host material 113Ha2 to that of the firstcarrier-transport compound 113H₁ and/or that of the secondcarrier-transport compound 113H₂; in this manner, the secondphosphorescent compound 113Db can emit light.

In this manner, the transfer of triplet excitation energy which accountsfor 75% of the excitation energy generated by recombination of carriersis taken into consideration; thus, light emission from the secondphosphorescent compound 113Db can be obtained with desired intensity.

In the case where the singlet excitation energy of the first hostmaterial 113Ha1 or the second host material 113Ha2 is higher than thesingle excitation energies of the first carrier-transport compound 113H₁and the second carrier-transport compound 113H₂, energy transfer occursdue to Dexter mechanism. Here, when the first host material 113Ha1 orthe second host material 113Ha2 is a fluorescent light-emittingmaterial, energy transfer occurs also owing to Förster mechanism.

Here, energy transfer to the phosphorescent compound for obtaining alight-emitting element having higher emission efficiency will bedescribed. In the following description, a substance providing aphosphorescent compound with energy is referred to as a host material.

Carrier recombination occurs in both the host material and thephosphorescent compound; thus, efficient energy transfer from the hostmaterial to the phosphorescent compound is needed to increase emissionefficiency. As mechanisms of the energy transfer from the host materialto the phosphorescent compound, two mechanisms have been proposed: oneis Dexter mechanism, and the other is Förster mechanism.

The efficiency of energy transfer from a host molecule to a guestmolecule (energy transfer efficiency Φ_(ET)) is expressed by thefollowing formula. In the formula, k_(r) denotes the rate constant of alight-emission process (fluorescence in energy transfer from a singletexcited state, and phosphorescence in energy transfer from a tripletexcited state), k_(n) denotes the rate constant of a non-light-emissionprocess (thermal deactivation or intersystem crossing), and τ denotesthe measured lifetime of an excited state.

$\begin{matrix}{\Phi_{ET} = {\frac{k_{h^{*}\rightarrow g}}{k_{r} + k_{n} + k_{h^{*}\rightarrow g}} = \frac{k_{h^{*}\rightarrow g}}{\left( \frac{1}{\tau} \right) + k_{h^{*}\rightarrow g}}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

First, according to the above formula, it is understood that the energytransfer efficiency Φ_(ET) can be increased by significantly increasingthe rate constant k_(h*→g) of energy transfer as compared with anothercompeting rate constant k_(r)+k_(n)(=1/τ). Then, in order to increasethe rate constant k_(h*→g) of energy transfer, in Förster mechanism andDexter mechanism, it is preferable that an emission spectrum of a hostmolecule (a fluorescent spectrum in energy transfer from a singletexcited state, and a phosphorescent spectrum in energy transfer from atriplet excited state) largely overlap with an absorption spectrum of aguest molecule (the phosphorescent compound in the second light-emittinglayer).

Here, the absorption band on the longest wavelength side (lowest energyside) in the absorption spectrum of the phosphorescent compound is ofimportance in considering the overlap between the emission spectrum ofthe host molecule and the absorption spectrum of the phosphorescentcompound.

In an absorption spectrum of the phosphorescent compound, an absorptionband that is considered to contribute to light emission most greatly isat an absorption wavelength corresponding to direct transition from aground state to a triplet excitation state and a vicinity of theabsorption wavelength, which is on the longest wavelength side. Fromthese considerations, it is preferable that the emission spectrum (afluorescent spectrum and a phosphorescent spectrum) of the host materialoverlap with the absorption band on the longest wavelength side in theabsorption spectrum of the phosphorescent compound.

For example, most organometallic complexes, especially light-emittingiridium complexes, have a broad absorption band at around 500 nm to 600nm as the absorption band on the longest wavelength side. Thisabsorption band is mainly based on a triplet MLCT (metal to ligandcharge transfer) transition. Note that it is considered that theabsorption band also includes absorptions based on a triplet π-π*transition and a singlet MLCT transition, and that these absorptionsoverlap with each other to form a broad absorption band on the longestwavelength side in the absorption spectrum. Therefore, when anorganometallic complex (especially iridium complex) is used as the guestmaterial, it is preferable to make the broad absorption band on thelongest wavelength side largely overlap with the emission spectrum ofthe host material as described above.

Here, first, energy transfer from a host material in a triplet excitedstate will be considered. From the above-described discussion, it ispreferable that, in energy transfer from a triplet excited state, thephosphorescent spectrum of the host material and the absorption band onthe longest wavelength side of the phosphorescent compound largelyoverlap with each other.

However, a question here is energy transfer from the host molecule inthe singlet excited state. In order to efficiently perform not onlyenergy transfer from the triplet excited state but also energy transferfrom the singlet excited state, it is clear from the above-describeddiscussion that the host material needs to be designed such that notonly its phosphorescent spectrum but also its fluorescent spectrumoverlaps with the absorption band on the longest wavelength side of theguest material. In other words, unless the host material is designed soas to have its fluorescent spectrum in a position similar to that of itsphosphorescent spectrum, it is not possible to achieve efficient energytransfer from the host material in both the singlet excited state andthe triplet excited state.

However, in general, the singlet excitation level differs greatly fromthe triplet excitation level (singlet excitation level>tripletexcitation level); therefore, the fluorescence wavelength also differsgreatly from the phosphorescence wavelength (fluorescencewavelength<phosphorescence wavelength). For example,4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP), which is commonlyused in a light-emitting element including a phosphorescent compound,has a phosphorescent spectrum at around 500 nm and has a fluorescentspectrum at around 400 nm, which are largely different by about 100 nm.This example also shows that it is extremely difficult to design a hostmaterial so as to have its fluorescent spectrum in a position similar tothat of its phosphorescent spectrum.

The singlet excitation energy level of one substance is higher than thetriplet excitation energy level thereof, and thus the triplet excitationlevel of a host material whose fluorescence spectrum corresponds to awavelength close to an absorption spectrum of a guest material on thelongest wavelength side is lower than the triplet excitation level ofthe guest material.

Here, an exciplex formed from two kinds of materials is described.Fluorescence from the exciplex is derived from the energy differencebetween the higher HOMO level of one of the two kinds of materials andthe lower LUMO level of the other material; thus, the fluorescencespectrum of the exciplex is on the longer wavelength side than that ofeither one of the two kinds of materials. For such a reason, energytransfer from a single excited state can be maximized while the tripletexcitation levels of the two kinds of compounds forming the exciplex arekept higher than the triplet excitation level of the guest material.

In addition, the exciplex is in a state where the triplet excitationlevel and the singlet excitation level are close to each other;therefore, the fluorescence spectrum and the phosphorescence spectrumexist at substantially the same position. Accordingly, both thefluorescence spectrum and the phosphorescence spectrum of the exciplexcan overlap largely with an absorption corresponding to transition ofthe guest molecule from the singlet ground state to the triplet excitedstate (a broad absorption band of the guest molecule existing on thelongest wavelength side in the absorption spectrum), and thus alight-emitting element having high energy transfer efficiency can beobtained.

In this manner, as a combination of the first carrier-transport compound113H₁ and the second carrier-transport compound 113H₂ in the secondlight-emitting layer, the one with which an exciplex is formed ispreferable. Further, when the absorption band of the secondphosphorescent compound 113Db on the lowest energy side and the emissionspectrum of the exciplex overlap with each other, the emissionefficiency of the light-emitting element can be higher. It is preferablethat the difference in equivalent energy value between a peak wavelengthin the absorption band of the second phosphorescent compound 113Db onthe lowest energy side and a peak wavelength of the emission spectrum ofthe exciplex be 0.2 eV or less in order that the spectra largely overlapwith each other.

In the case of the structure illustrated in FIG. 1B, as a combination ofthe first host material 113Ha1 and the second host material 113Ha2, theone with which an exciplex is formed is preferable. Further, when theabsorption band of the first phosphorescent compound on the lowestenergy side and the emission spectrum of the exciplex overlap with eachother, the emission efficiency of the light-emitting element can behigher. It is preferable that the difference in equivalent energy valuebetween a peak wavelength in the absorption band of the firstphosphorescent compound 113Da on the lowest energy side and a peakwavelength of the emission spectrum of the exciplex be 0.2 eV or less inorder that the spectra largely overlap with each other.

Light emission from the exciplex is, as described above, derived fromthe energy difference between the higher HOMO level of one of the twokinds of materials forming the exciplex and the lower LUMO level of theother material. For such a reason, with the use of the exciplex as ahost, a change in the combination of materials can change the emissionspectrum; thus, the emission spectrum can be easily adjusted to overlapwith the absorption of the phosphorescent compound on the longwavelength side.

Further, the first host material 113Ha1 and the second host material113Ha2 preferably have higher triplet excitation energy than the firstphosphorescent compound 113Da in order that light emission from thefirst phosphorescent compound 113Da is not quenched. Furthermore, thefirst carrier-transport compound 113H₁ and the second carrier-transportcompound 113H₂ preferably have higher triplet excitation energy than thesecond phosphorescent compound 113Db in order that light emission fromthe second phosphorescent compound 113Db is not quenched.

A light-emitting element having the above structure can have highemission efficiency. Further, light emission from phosphorescentcompounds in the light-emitting element can be provided in a balancedmanner.

The following compounds are examples of an electron-transport compoundand a hole-transport compound that can be used for the first hostmaterial 113Ha1, the second host material 113Ha2, the firstcarrier-transport compound 113H₁, and the second carrier-transportcompound 113H₂. Note that one of the first host material 113Ha1 and thesecond host material 113Ha2 is an electron-transport compound, and theother of them is a hole-transport compound. Similarly, one of the firstcarrier-transport compound 113H₁ and the second carrier-transportcompound 113H₂ is an electron-transport compound, and the other of themis a hole-transport compound. As combinations thereof, the ones withwhich exciplexes are formed are preferable.

The following are examples of the electron-transport compound: a metalcomplex such as bis(10-hydroxybenzo[h]quinolinato)beryllium(II)(abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq),bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), orbis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ); aheterocyclic compound having a polyazole skeleton such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ),1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation:CO11), 2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI), or2-[3-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole(abbreviation: mDBTBIm-II); a heterocyclic compound having a diazineskeleton such as2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II),2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II),2-[3′-(9H-carbazol-9-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mCzBPDBq), 4,6-bis[3-(phenanthren-9-yl)phenyl]pyrimidine(abbreviation: 4,6mPnP2Pm), or4,6-bis[3-(4-dibenzothienyl)phenyl]pyrimidine (abbreviation:4,6mDBTP2Pm-II); and a heterocyclic compound having a pyridine skeletonsuch as 3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (abbreviation:35DCzPPy) or 1,3,5-tri[(3-pyridyl)phen-3-yl]benzene (abbreviation:TmPyPB). In particular, a π-electron deficient heteroaromatic compoundis preferable. Among the above materials, a heterocyclic compound havinga diazine skeleton and a heterocyclic compound having a pyridineskeleton have high reliability and are thus preferable. Specifically, aheterocyclic compound having a diazine (pyrimidine or pyrazine) skeletonhas a high electron-transport property to contribute to a reduction indrive voltage.

The following are examples of the hole-transport compound: a compoundhaving an aromatic amine skeleton such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD),4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP), 4-phenyl-3′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: mBPAFLP),4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBA1BP), 4,4′-diphenyl-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBBi1BP),4-(1-naphthyl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBANB),4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBNBB),9,9-dimethyl-N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]fluoren-2-amine(abbreviation: PCBAF), orN-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]spiro-9,9′-bifluoren-2-amine(abbreviation: PCBASF); a compound having a carbazole skeleton such as1,3-bis(N-carbazolyl)benzene (abbreviation: mCP),4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),3,6-bis(3,5-diphenylphenyl)-9-phenylcarbazole (abbreviation: CzTP), or3,3′-bis(9-phenyl-9H-carbazole) (abbreviation: PCCP); a compound havinga thiophene skeleton such as4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II),2,8-diphenyl-4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]dibenzothiophene(abbreviation: DBTFLP-III), or4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-6-phenyldibenzothiophene(abbreviation: DBTFLP-IV); and a compound having a furan skeleton suchas 4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzofuran) (abbreviation:DBF3P-II) or4-{3-[3-(9-phenyl-9H-fluoren-9-yl)phenyl]phenyl}dibenzofuran(abbreviation: mmDBFFLBi-II). In particular, a π-electron richheteroaromatic compound is preferable. Among the above materials, acompound having an aromatic amine skeleton and a compound having acarbazole skeleton are preferable because these compounds are highlyreliable and have high hole-transport properties to contribute to areduction in drive voltage.

A light-emitting element having the above structure has high emissionefficiency. Further, light emission from a plurality of light-emittingsubstances in the light-emitting element can be obtained. Thelight-emitting element does not have a tandem structure, and thus itsmanufacturing process is not complicated and the amount of power lossdue to an intermediate layer is small. In addition, the light-emittingelement has a high utility value as a white light-emitting element.

As illustrated in FIGS. 1C and 1D, the light-emitting layer 113 may havea three-layer structure of the first light-emitting layer 113 a, thesecond light-emitting layer 113 b, and a third light-emitting layer 113c. In this case, the relation between the first light-emitting layer 113a and the second light-emitting layer 113 b is as described above.

The third light-emitting layer 113 c includes a third phosphorescentcompound 113Dc, a third carrier-transport compound 113H₃, and a fourthcarrier-transport compound 113H₄. The emission wavelength of the thirdphosphorescent compound 113Dc is longer than that of the secondphosphorescent compound 113Db. One of the third carrier-transportcompound 113H₃ and the fourth carrier-transport compound 113H₄ is anelectron-transport compound, and the other of them is a hole-transportcompound. As examples of compounds that can be used as the thirdcarrier-transport compound 113H₃ and the fourth carrier-transportcompound 113H₄, the above compounds that can be used as the first hostmaterial 113Ha1, the second host material 113Ha2, the firstcarrier-transport compound 113H₁, and the second carrier-transportcompound 113H₂ can be given.

Similarly to the first carrier-transport compound 113H₁ and the secondcarrier-transport compound 113H₂, or the first host material 113Ha1 andthe second host material 113Ha2, the third carrier-transport compound113H₃ and the fourth carrier-transport compound 113H₄ preferably form anexciplex. Further, the emission spectrum of the exciplex and theabsorption band of the third phosphorescent compound 113Dc on thelongest wavelength side preferably overlap with each other in order thatenergy transfer from the exciplex to the third phosphorescent compound113Dc is optimized. It is preferable that the difference in equivalentenergy value between a peak wavelength in the absorption band of thethird phosphorescent compound 113Dc on the lowest energy side and a peakwavelength of the emission spectrum of the exciplex be 0.2 eV or less inorder that the spectra largely overlap with each other. Further, thethird carrier-transport compound 113H₃ and the fourth carrier-transportcompound 113H₄ preferably have higher triplet excitation energy than thethird phosphorescent compound 113Dc in order that light emission fromthe third phosphorescent compound 113Dc is not quenched.

The third light-emitting layer 113 c emits light in such a manner thatrecombination energy generated in the recombination region in the firstlight-emitting layer 113 a or in the vicinity of the interface betweenthe first light-emitting layer 113 a and the second light-emitting layer113 b is transferred to the third light-emitting layer 113 c through thesecond light-emitting layer 113 b. Therefore, the triplet excitationenergies of the first carrier-transport compound 113H₁ and the secondcarrier-transport compound 113H₂ are preferably higher than the tripletexcitation energy of one of the third carrier-transport compound 113H₃and the fourth carrier-transport compound 113H₄.

The third light-emitting layer 113 c preferably has the samecarrier-transport property as the second light-emitting layer 113 b inorder that a recombination region is in the first light-emitting layer113 a or in the vicinity of the interface between the firstlight-emitting layer 113 a and the second light-emitting layer 113 b.Further, one of or both the electron-transport compound and thehole-transport compound in the third carrier-transport compound 113H₃and the fourth carrier-transport compound 113H₄ may be the same as oneof or both the electron-transport compound and the hole-transportcompound in the first carrier-transport compound 113H₁ and the secondcarrier-transport compound 113H₂. In this case, the materials are usedin common between different layers, which is advantageous in cost.

In the light-emitting element in FIG. 1E including the light-emittinglayer 113 illustrated in FIG. 1C or 1D, the first light-emitting layer113 a may be formed on the anode side or cathode side.

In the case where the first light-emitting layer 113 a is formed on theanode side, the first light-emitting layer 113 a is preferably a layerhaving a hole-transport property and the second light-emitting layer 113b and the third light-emitting layer 113 c are preferably layers eachhaving an electron-transport property. In the first light-emittinglayer, the first host material 113Ha1 may have a hole-transport propertyin the case of the structure illustrated in FIG. 1C. Thecarrier-transport property of the first light-emitting layer 113 a canbe adjusted by changing the mixture ratio of the first host material113Ha1 to the second host material 113Ha2 (that is, theelectron-transport compound to the hole-transport compound) in the caseof the structure illustrated in FIG. 1D. In a similar manner, the secondlight-emitting layer 113 b and the third light-emitting layer 113 c canhave desired carrier-transport properties by changing the mixture ratioof the first carrier-transport compound 113H₁ to the secondcarrier-transport compound 113H₂, the third carrier-transport compound113H₃, and the fourth carrier-transport compound 113H₄.

In the case where the first light-emitting layer 113 a is formed on thecathode side, the first light-emitting layer 113 a is preferably a layerhaving an electron-transport property and the second light-emittinglayer 113 b and the third light-emitting layer 113 c are preferablylayers each having a hole-transport property. In the firstlight-emitting layer 113 a, the first host material 113Ha1 may have anelectron-transport property in the case of the structure illustrated inFIG. 1C. The carrier-transport property of the first light-emittinglayer 113 a can be adjusted by changing the mixture ratio of the firsthost material 113Ha1 to the second host material 113Ha2 (that is, theelectron-transport compound to the hole-transport compound) in the caseof the structure illustrated in FIG. 1D. In a similar manner, the secondlight-emitting layer 113 b and the third light-emitting layer 113 c canhave desired carrier-transport properties by changing the mixture ratioof the first carrier-transport compound 113H₁ to the secondcarrier-transport compound 113H₂, the third carrier-transport compound113H₃, and the fourth carrier-transport compound 113H₄.

Energy is transferred through the second light-emitting layer 113 b, andthus is not transferred to the third light-emitting layer 113 c when thethickness of the second light-emitting layer 113 b is too large; in thiscase, light emission from the third phosphorescent compound 113Dc cannotbe obtained. Therefore, in order that light emission from the thirdlight-emitting layer 113 c is obtained, the thickness of the secondlight-emitting layer 113 b is preferably greater than or equal to 2 nmand less than or equal to 20 nm, more preferably greater than or equalto 5 nm and less than or equal to 10 nm.

In the light-emitting element of this embodiment including thelight-emitting layer 113 illustrated in FIG. 1C or 1D, a compoundexhibiting blue light emission, a compound exhibiting green lightemission, and a compound exhibiting red light emission are used as thefirst phosphorescent compound 113Da, the second phosphorescent compound113Db, and the third phosphorescent compound 113Dc, respectively; thus,favorable white light emission (e.g., white light emission that meetsthe standards defined by Japanese Industrial Standards (JIS)) can beobtained. Such white light emission has an excellent color renderingproperty. Such a white light-emitting element is significantly suitablefor lighting.

A light-emitting element having the above structure includes a pluralityof light-emitting substances and has high emission efficiency. Further,light emission from the plurality of light-emitting substances in thelight-emitting element can be provided in a balanced manner.

Embodiment 2

In this embodiment, a detailed example of the structure of thelight-emitting element described in Embodiment 1 will be described belowwith reference to FIG. 1E.

A light-emitting element in this embodiment includes, between a pair ofelectrodes, an EL layer including a plurality of layers. In thisembodiment, the light-emitting element includes the first electrode 101,the second electrode 102, and the EL layer 103 which is provided betweenthe first electrode 101 and the second electrode 102. The followingdescription in this embodiment is made on the assumption that the firstelectrode 101 functions as an anode and the second electrode 102functions as a cathode. In other words, when a voltage is appliedbetween the first electrode 101 and the second electrode 102 so that thepotential of the first electrode 101 is higher than that of the secondelectrode 102, light emission can be obtained.

Since the first electrode 101 functions as the anode, the firstelectrode 101 is preferably formed using any of metals, alloys,conductive compounds with a high work function (specifically, a workfunction of 4.0 eV or more), mixtures thereof, and the like.Specifically, for example, indium oxide-tin oxide (ITO: indium tinoxide), indium oxide-tin oxide containing silicon or silicon oxide,indium oxide-zinc oxide, indium oxide containing tungsten oxide and zincoxide (IWZO), and the like can be given. Films of these conductive metaloxides are usually formed by a sputtering method but may be formed byapplication of a sol-gel method or the like. In an example of theformation method, indium oxide-zinc oxide is deposited by a sputteringmethod using a target obtained by adding 1 wt % to 20 wt % of zinc oxideto indium oxide. Further, a film of indium oxide containing tungstenoxide and zinc oxide (IWZO) can be formed by a sputtering method using atarget in which tungsten oxide and zinc oxide are added to indium oxideat 0.5 wt % to 5 wt % and 0.1 wt % to 1 wt %, respectively. Besides,gold (Au), platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr),molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd),nitrides of metal materials (e.g., titanium nitride), and the like canbe given. Graphene can also be used. Note that when a composite materialdescribed later is used for a layer which is in contact with the firstelectrode 101 in the EL layer 103, an electrode material can be selectedregardless of its work function.

There is no particular limitation on the stacked-layer structure of theEL layer 103 as long as the light-emitting layer 113 has the structuredescribed in Embodiment 1. For example, the EL layer 103 can be formedby combining a hole-injection layer, a hole-transport layer, thelight-emitting layer, an electron-transport layer, an electron-injectionlayer, a carrier-blocking layer, and the like as appropriate. In thisembodiment, the EL layer 103 has a structure in which the hole-injectionlayer 111, the hole-transport layer 112, the light-emitting layer 113,the electron-transport layer 114, and the electron-injection layer 115are stacked in this order over the first electrode 101. Specificexamples of materials used for each layer are given below.

The hole-injection layer 111 is a layer containing a substance having ahigh hole-injection property. Molybdenum oxide, vanadium oxide,ruthenium oxide, tungsten oxide, manganese oxide, or the like can beused. Alternatively, the hole-injection layer 111 can be formed using aphthalocyanine-based compound such as phthalocyanine (abbreviation:H₂Pc) or copper phthalocyanine (abbreviation: CuPc), an aromatic aminecompound such as4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB) orN,N′-bis{4-[bis(3-methylphenyl)amino]phenyl}-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine(abbreviation: DNTPD), a high molecular compound such aspoly(ethylenedioxythiophene)/poly(styrenesulfonic acid) (abbreviation:PEDOT/PSS), or the like.

Alternatively, a composite material in which a substance having ahole-transport property contains a substance having an acceptor propertycan be used for the hole-injection layer 111. Note that the use of sucha substance having a hole-transport property which contains a substancehaving an acceptor property enables selection of a material used to forman electrode regardless of its work function. In other words, besides amaterial having a high work function, a material having a low workfunction can also be used for the first electrode 101. As the substancehaving an acceptor property,7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ), chloranil, and the like can be given. In addition, transitionmetal oxides can be given. Oxides of the metals that belong to Groups 4to 8 of the periodic table can be given. Specifically, vanadium oxide,niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide,tungsten oxide, manganese oxide, and rhenium oxide are preferable inthat their electron-accepting property is high. Among these oxides,molybdenum oxide is particularly preferable in that it is stable in theair, has a low hygroscopic property, and is easy to handle.

As the substance having a hole-transport property which is used for thecomposite material, any of a variety of organic compounds such asaromatic amine compounds, carbazole derivatives, aromatic hydrocarbons,and high molecular compounds (e.g., oligomers, dendrimers, or polymers)can be used. Note that the organic compound used for the compositematerial is preferably an organic compound having a high hole-transportproperty. Specifically, a substance having a hole mobility of 10⁻⁶cm²/Vs or more is preferably used. Organic compounds that can be used asthe substance having a hole-transport property in the composite materialare specifically given below.

Examples of the aromatic amine compounds areN,N′-di(p-tolyl)-N,N′-diphenyl-p-phenylenediamine (abbreviation:DTDPPA), 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl(abbreviation: DPAB),N,N′-bis{4-[bis(3-methylphenyl)amino]phenyl}-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine(abbreviation: DNTPD),1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B), and the like.

Specific examples of the carbazole derivatives that can be used for thecomposite material are3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2),3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1), and the like.

Other examples of the carbazole derivatives that can be used for thecomposite material are 4,4′-di(N-carbazolyl)biphenyl (abbreviation:CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB),9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA),1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene, and thelike.

Examples of the aromatic hydrocarbons that can be used for the compositematerial are 2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation:t-BuDNA), 2-tert-butyl-9,10-di(1-naphthyl)anthracene,9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA),2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA),9,10-di(2-naphthyl)anthracene (abbreviation: DNA),9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene(abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene(abbreviation: DMNA),2-tert-butyl-9,10-bis[2-(1-naphthyl)phenyl]anthracene,9,10-bis[2-(1-naphthyl)phenyl]anthracene,2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene,2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl,10,10′-diphenyl-9,9′-bianthryl,10,10′-bis(2-phenylphenyl)-9,9′-bianthryl,10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene,tetracene, rubrene, perylene, 2,5,8,11-tetra(tert-butyl)perylene, andthe like. Besides, pentacene, coronene, or the like can also be used.The aromatic hydrocarbon which has a hole mobility of 1×10⁻⁶ cm²/Vs ormore and which has 14 to 42 carbon atoms is particularly preferable.

Note that the aromatic hydrocarbons that can be used for the compositematerial may have a vinyl skeleton. Examples of the aromatic hydrocarbonhaving a vinyl group are 4,4′-bis(2,2-diphenylvinyl)biphenyl(abbreviation: DPVBi), 9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene(abbreviation: DPVPA), and the like.

A polymeric compound such as poly(N-vinylcarbazole) (abbreviation: PVK)poly(4-vinyltriphenylamine) (abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA), orpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation:poly-TPD) can also be used.

By providing a hole-injection layer, a high hole-injection property canbe achieved to allow a light-emitting element to be driven at a lowvoltage.

The hole-transport layer 112 is a layer that contains a substance havinga hole-transport property. Examples of the substance having ahole-transport property are aromatic amine compounds such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(N,N′-diphenylamino)triphenylamine(abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA),4,4′-bis[N-(Spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP), and the like. The substances mentioned here havehigh hole-transport properties and are mainly ones that have a holemobility of 10⁻⁶ cm²/Vs or more. An organic compound given as an exampleof the substance having a hole-transport property in the compositematerial described above can also be used for the hole-transport layer112. A polymeric compound such as poly(N-vinylcarbazole) (abbreviation:PVK) or poly(4-vinyltriphenylamine) (abbreviation: PVTPA) can also beused. Note that the layer that contains a substance having ahole-transport property is not limited to a single layer, and may be astack of two or more layers including any of the above substances.

The light-emitting layer 113 has the structure described inEmbodiment 1. Therefore, the light-emitting element of this embodimenthas high emission efficiency, and light emission from a plurality ofphosphorescent compounds in the light-emitting element can be providedin a balanced manner. Embodiment 1 is to be referred to for the mainstructure of the light-emitting layer 113.

There is no particular limitation on materials that can be used as thefirst to third phosphorescent compounds 113Da to 113Dc in thelight-emitting layer 113 as long as they have the relation described inEmbodiment 1. The following can be given as examples of the first tothird phosphorescent compounds 113Da to 113Dc.

The following are the specific examples: an organometallic iridiumcomplex having a 4H-triazole skeleton, such astris{2-[5-(2-methylphenyl)-4-(2,6-dimethylphenyl)-4H-1,2,4-triazol-3-yl-κN2]phenyl-κC}iridium(III)(abbreviation: Ir(mpptz-dmp)₃),tris(5-methyl-3,4-diphenyl-4H-1,2,4-triazolato)iridium(III)(abbreviation: Ir(Mptz)₃), ortris[4-(3-biphenyl)-5-isopropyl-3-phenyl-4H-1,2,4-triazolato]iridium(III)(abbreviation: Ir(iPrptz-3b)₃); an organometallic iridium complex havinga 1H-triazole skeleton, such astris[3-methyl-1-(2-methylphenyl)-5-phenyl-1H-1,2,4-triazolato]iridium(III)(abbreviation: Ir(Mptz1-mp)₃) ortris(1-methyl-5-phenyl-3-propyl-1H-1,2,4-triazolato)iridium(III)(abbreviation: Ir(Prptz1-Me)₃); an organometallic iridium complex havingan imidazole skeleton, such asfac-tris[1-(2,6-diisopropylphenyl)-2-phenyl-1H-imidazole]iridium(III)(abbreviation: Ir(iPrpmi)₃), ortris[3-(2,6-dimethylphenyl)-7-methylimidazo[1,2-f]phenanthridinato]iridium(III)(abbreviation: Ir(dmpimpt-Me)₃); and an organometallic iridium complexin which a phenylpyridine derivative having an electron-withdrawinggroup is a ligand, such as bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)tetrakis(1-pyrazolyl)borate (abbreviation: FIr6),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III) picolinate(abbreviation: Flrpic),bis[2-(3′,5′-bistrifluoromethylphenyl)pyridinato-N,C^(2′)]iridium(III)picolinate(abbreviation: Ir(CF₃ppy)₂(pic)), orbis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)acetylacetonate (abbreviation: FIracac). These are compounds emittingblue phosphorescence and have an emission peak at 440 nm to 520 nm.Among the above compounds, an organometallic iridium complex having apolyazole skeleton such as a 4H-triazole skeleton, a 1H-triazoleskeleton, or an imidazole skeleton has a high hole-trapping property.Therefore, it is preferable that any of these compounds be used as thefirst phosphorescent compound in the light-emitting element of oneembodiment of the present invention, the first light-emitting layer beprovided closer to the cathode than the second light-emitting layer, andthe second light-emitting layer have a hole-transport property(specifically, the second host material be a hole-transport material),in which case a recombination region of carriers can be easilycontrolled to be in the first light-emitting layer. Note that anorganometallic iridium complex having a 4H-triazole skeleton hasexcellent reliability and emission efficiency and thus is especiallypreferable.

The following are the specific examples: an organometallic iridiumcomplex having a pyrimidine skeleton, such astris(4-methyl-6-phenylpyrimidinato)iridium(III) (abbreviation:Ir(mppm)₃), tris(4-t-butyl-6-phenylpyrimidinato)iridium(III)(abbreviation: Ir(tBuppm)₃),(acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium(III)(abbreviation: Ir(mppm)₂(acac)),(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)(abbreviation: Ir(tBuppm)₂(acac)),(acetylacetonato)bis[6-(2-norbornyl)-4-phenylpyrimidinato]iridium(III)(abbreviation: Ir(nbppm)₂(acac)),(acetylacetonato)bis[5-methyl-6-(2-methylphenyl)-4-phenylpyrimidinato]iridium(III)(abbreviation: Ir(mpmppm)₂(acac)), or(acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III)(abbreviation: Ir(dppm)₂(acac)); an organometallic iridium complexhaving a pyrazine skeleton, such as(acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III)(abbreviation: Ir(mppr-Me)₂(acac)) or(acetylacetonato)bis(5-isopropyl-3-methyl-2-phenylpyrazinato)iridium(III)(abbreviation: Ir(mppr-iPr)₂(acac)); an organometallic iridium complexhaving a pyridine skeleton, such astris(2-phenylpyridinato-N,C^(2′))iridium(III) (abbreviation: Ir(ppy)₃),bis(2-phenylpyridinato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(ppy)₂acac),bis(benzo[h]quinolinato)iridium(III)acetylacetonate (abbreviation:Ir(bzq)₂(acac)), tris(benzo[h]quinolinato)iridium(III) (abbreviation:Ir(bzq)₃), tris(2-phenylquinolinato-N,C^(2′))iridium(III) (abbreviation:Ir(pq)₃), orbis(2-phenylquinolinato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(pq)₂(acac)); and a rare earth metal complex such astris(acetylacetonato)(monophenanthroline)terbium(III) (abbreviation:Tb(acac)₃(Phen)). These are mainly compounds emitting greenphosphorescence and have an emission peak at 500 nm to 600 nm. Among theabove compounds, an organometallic iridium complex having a diazineskeleton such as a pyrimidine skeleton or a pyrazine skeleton has a lowhole-trapping property and a high electron-trapping property. Therefore,it is preferable that any of these compounds be used as the firstphosphorescent compound in the light-emitting element of one embodimentof the present invention, the first light-emitting layer be providedcloser to the anode than the second light-emitting layer, and the secondlight-emitting layer have an electron-transport property (specifically,the second host material be an electron-transport material), in whichcase a recombination region of carriers can be easily controlled to bein the first light-emitting layer. Note that an organometallic iridiumcomplex having a pyrimidine skeleton has distinctively high reliabilityand emission efficiency and thus is especially preferable.

Still other examples are an organometallic iridium complex having apyrimidine skeleton such as(diisobutyrylmethanato)bis[4,6-bis(3-methylphenyl)pyrimidinato]iridium(III)(abbreviation: Ir(5mdppm)₂(dibm)),bis[4,6-bis(3-methylphenyl)pyrimidinato](dipivaloylmethanato)iridium(III)(abbreviation: Ir(5mdppm)₂(dpm)), orbis[4,6-di(naphthalen-1-yl)pyrimidinato](dipivaloylmethanato)iridium(III)(abbreviation: Ir(d1npm)₂(dpm)); an organometallic iridium complexhaving a pyrazine skeleton such as(acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III)(abbreviation: Ir(tppr)₂(acac)),bis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III)(abbreviation: Ir(tppr)₂(dpm)), or(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: Ir(Fdpq)₂(acac)); an organometallic iridium complexhaving a pyridine skeleton such astris(1-phenylisoquinolinato-N,C^(2′))iridium(III) (abbreviation:Ir(piq)₃) or bis(1-phenylisoquinolinato-N,C^(2′))iridium(III)acetylacetonate (abbreviation: Ir(piq)₂(acac)); a platinum complex suchas 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II)(abbreviation: PtOEP); and a rare earth metal complex such astris(1,3-diphenyl-1,3-propanedionato)(monophenanthroline)europium(III)(abbreviation: Eu(DBM)₃(Phen)) ortris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III)(abbreviation: Eu(TTA)₃(Phen)). These are compounds emitting redphosphorescence and have an emission peak at 600 nm to 700 nm. Among theabove compounds, an organometallic iridium complex having a diazineskeleton such as a pyrimidine skeleton or a pyrazine skeleton has a lowhole-trapping property and a high electron-trapping property. Therefore,it is preferable that an organometallic iridium complex having a diazineskeleton be used as the second phosphorescent compound, the firstlight-emitting layer be provided closer to the cathode than the secondlight-emitting layer, and the second light-emitting layer have ahole-transport property (specifically, the second host material be ahole-transport material), in which case a recombination region ofcarriers can be easily controlled to be in the first light-emittinglayer. Note that an organometallic iridium complex having a pyrimidineskeleton has distinctively high reliability and emission efficiency andthus is especially preferable. Further, because an organometalliciridium complex having a pyrazine skeleton can provide red lightemission with favorable chromaticity, the use of the organometalliciridium complex in a white light-emitting element improves a colorrendering property of the white light-emitting element.

It is also possible to select the first to third phosphorescentcompounds 113Da to 113Dc from known phosphorescent materials in additionto the above phosphorescent compounds.

Note that the phosphorescent compounds (the first to thirdphosphorescent compounds 113Da to 113Dc) may be replaced with a materialexhibiting thermally activated delayed fluorescence, i.e., a thermallyactivated delayed fluorescence (TADF) material. Here, the term “delayedfluorescence” refers to light emission having a spectrum similar tonormal fluorescence and an extremely long lifetime. The lifetime is 10⁻⁶seconds or longer, preferably 10⁻³ seconds or longer. Specific examplesof the thermally activated delayed fluorescence material include afullerene, a derivative thereof, an acridine derivative such asproflavine, and eosin. Other examples include a metal-containingporphyrin, such as a porphyrin containing magnesium (Mg), zinc (Zn),cadmium (Cd), tin (Sn), platinum (Pt), indium (In), or palladium (Pd).Examples of the metal-containing porphyrin include a protoporphyrin-tinfluoride complex (SnF₂(Proto IX)), a mesoporphyrin-tin fluoride complex(SnF₂(Meso IX)), a hematoporphyrin-tin fluoride complex (SnF₂(HematoIX)), a coproporphyrin tetramethyl ester-tin fluoride complex(SnF₂(Copro III-4Me)), an octaethylporphyrin-tin fluoride complex(SnF₂(OEP)), an etioporphyrin-tin fluoride complex (SnF₂(Etio I)), andan octaethylporphyrin-platinum chloride complex (PtCl₂OEP).Alternatively, a heterocyclic compound including a π-electron richheteroaromatic ring and a π-electron deficient heteroaromatic ring canbe used, such as2-(biphenyl-4-yl)-4,6-bis(12-phenylindolo[2,3-a]carbazol-11-yl)-1,3,5-triazine(PIC-TRZ). Note that a material in which the π-electron richheteroaromatic ring is directly bonded to the π-electron deficientheteroaromatic ring is particularly preferably used because both thedonor property of the π-electron rich heteroaromatic ring and theacceptor property of the π-electron deficient heteroaromatic ring areincreased and the energy difference between the S₁ level and the T₁level becomes small.

Materials that can be used for the first host material 113Ha1, thesecond host material 113Ha2, the first carrier-transport compound 113H₁,the second carrier-transport compound 113H₂, the third carrier-transportcompound 113H₃, and the fourth carrier-transport compound 113H₄ aredescribed in Embodiment 1; thus, the description thereof is not givenhere.

For formation of the light-emitting layer 113, co-evaporation by avacuum evaporation method can be used, or alternatively an inkjetmethod, a spin coating method, a dip coating method, or the like using amixed solution can be used.

The electron-transport layer 114 is a layer containing a substancehaving an electron-transport property. For example, a layer containing ametal complex having a quinoline skeleton or a benzoquinoline skeleton,such as tris(8-quinolinolato)aluminum (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium (abbreviation: BeBq₂), orbis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (abbreviation:BAlq), or the like can be used. Alternatively, a metal complex having anoxazole-based or thiazole-based ligand, such asbis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation: Zn(BOX)₂) orbis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation: Zn(BTZ)₂), orthe like can be used. Besides the metal complexes,2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP), or the like can also be used. Thesubstances mentioned here have high electron-transport properties andare mainly ones that have an electron mobility of 10⁻⁶ cm²/Vs or more.Note that any of the above-described host materials havingelectron-transport properties may be used for the electron-transportlayer 114.

The electron-transport layer 114 is not limited to a single layer, andmay be a stack of two or more layers containing any of the abovesubstances.

Between the electron-transport layer and the light-emitting layer, alayer that controls transport of electron carriers may be provided. Thisis a layer formed by addition of a small amount of a substance having ahigh electron-trapping property to the aforementioned material having ahigh electron-transport property, and the layer is capable of adjustingcarrier balance by retarding transport of electron carriers. Such astructure is very effective in preventing a problem (such as a reductionin element lifetime) caused when electrons pass through thelight-emitting layer.

In addition, the electron-injection layer 115 may be provided in contactwith the second electrode 102 between the electron-transport layer 114and the second electrode 102. For the electron-injection layer 115, analkali metal, an alkaline earth metal, or a compound thereof, such aslithium fluoride (LiF), cesium fluoride (CsF), or calcium fluoride(CaF₂), can be used. For example, a layer that is formed using asubstance having an electron-transport property and contains an alkalimetal, an alkaline earth metal, or a compound thereof can be used. Notethat a layer that is formed using a substance having anelectron-transport property and contains an alkali metal or an alkalineearth metal is preferably used as the electron-injection layer 115, inwhich case electron injection from the second electrode 102 isefficiently performed.

For the second electrode 102, any of metals, alloys, electricallyconductive compounds, and mixtures thereof which have a low workfunction (specifically, a work function of 3.8 eV or less) or the likecan be used. Specific examples of such a cathode material are elementsbelonging to Groups 1 and 2 of the periodic table, such as alkali metals(e.g., lithium (Li) and cesium (Cs)), magnesium (Mg), calcium (Ca), andstrontium (Sr), alloys thereof (e.g., MgAg and AlLi), rare earth metalssuch as europium (Eu) and ytterbium (Yb), alloys thereof, and the like.However, when the electron-injection layer is provided between thesecond electrode 102 and the electron-transport layer, for the secondelectrode 102, any of a variety of conductive materials such as Al, Ag,indium oxide-tin oxide, or indium oxide-tin oxide containing silicon orsilicon oxide can be used regardless of the work function. Films ofthese conductive materials can be formed by a sputtering method, aninkjet method, a spin coating method, or the like.

Any of a variety of methods can be used to form the EL layer 103regardless whether it is a dry process or a wet process. For example, avacuum evaporation method, an inkjet method, a spin coating method, orthe like may be used. Different formation methods may be used for theelectrodes or the layers.

In addition, the electrode may be formed by a wet method using a sol-gelmethod, or by a wet method using paste of a metal material.Alternatively, the electrode may be formed by a dry method such as asputtering method or a vacuum evaporation method.

In the light-emitting element having the above-described structure,current flows due to a potential difference between the first electrode101 and the second electrode 102, and holes and electrons recombine inthe light-emitting layer 113 which contains a substance having a highlight-emitting property, so that light is emitted. That is, alight-emitting region is formed in the light-emitting layer 113.

Light emission is extracted out through one of or both the firstelectrode 101 and the second electrode 102. Therefore, one of or boththe first electrode 101 and the second electrode 102 arelight-transmitting electrodes. In the case where only the firstelectrode 101 is a light-transmitting electrode, light emission isextracted through the first electrode 101. In the case where only thesecond electrode 102 is a light-transmitting electrode, light emissionis extracted through the second electrode 102. In the case where boththe first electrode 101 and the second electrode 102 arelight-transmitting electrodes, light emission is extracted through thefirst electrode 101 and the second electrode 102.

The structure of the layers provided between the first electrode 101 andthe second electrode 102 is not limited to the above-describedstructure. Preferably, a light-emitting region where holes and electronsrecombine is positioned away from the first electrode 101 and the secondelectrode 102 so that quenching due to the proximity of thelight-emitting region and a metal used for electrodes andcarrier-injection layers can be prevented.

Further, in order that transfer of energy from an exciton generated inthe light-emitting layer can be suppressed, preferably, thehole-transport layer and the electron-transport layer which are incontact with the light-emitting layer 113, particularly acarrier-transport layer in contact with a side closer to thelight-emitting region in the light-emitting layer 113, are formed usinga substance having higher triplet excitation energy than the substancein the light-emitting layer.

A light-emitting element in this embodiment is preferably fabricatedover a substrate of glass, plastic, or the like. As the way of stackinglayers over the substrate, layers may be sequentially stacked from thefirst electrode 101 side or sequentially stacked from the secondelectrode 102 side. In a light-emitting device, although onelight-emitting element may be formed over one substrate, a plurality oflight-emitting elements may be formed over one substrate. With aplurality of light-emitting elements as described above formed over onesubstrate, a lighting device in which elements are separated or apassive matrix light-emitting device can be manufactured. Alight-emitting element may be formed over an electrode electricallyconnected to a thin film transistor (TFT), for example, which is formedover a substrate of glass, plastic, or the like, so that an activematrix light-emitting device in which the TFT controls the drive of thelight-emitting element can be manufactured. Note that there is noparticular limitation on the structure of the TFT, which may be astaggered TFT or an inverted staggered TFT. In addition, crystallinityof a semiconductor used for the TFT is not particularly limited either;an amorphous semiconductor or a crystalline semiconductor may be used.In addition, a driver circuit formed in a TFT substrate may be formedwith an n-type TFT and a p-type TFT, or with either an n-type TFT or ap-type TFT.

Note that this embodiment can be combined with any of the otherembodiments as appropriate.

Embodiment 3

In this embodiment, an embodiment of a light-emitting element with astructure in which a plurality of light-emitting units are stacked(hereinafter, also referred to as “stacked-type element”) will bedescribed with reference to FIG. 2. This light-emitting element is alight-emitting element including a plurality of light-emitting unitsbetween a first electrode and a second electrode. One light-emittingunit has the same structure as the EL layer 103 illustrated in FIG. 1E.In other words, the light-emitting element illustrated in FIG. 1Eincludes one light-emitting unit while the light-emitting element inthis embodiment includes a plurality of light-emitting units.

In FIG. 2, a first light-emitting unit 511 and a second light-emittingunit 512 are stacked between a first electrode 501 and a secondelectrode 502, and a charge-generation layer 513 is provided between thefirst light-emitting unit 511 and the second light-emitting unit 512.The first electrode 501 and the second electrode 502 correspond,respectively, to the first electrode 101 and the second electrode 102 inFIG. 1E, and the materials given in the description with reference toFIG. 1E can be used. Further, the first light-emitting unit 511 and thesecond light-emitting unit 512 may have the same structure or differentstructures.

The charge-generation layer 513 contains a composite material of anorganic compound and a metal oxide. As this composite material of anorganic compound and a metal oxide, the composite material that can beused for the hole-injection layer 111 illustrated in FIG. 1E can beused. As the organic compound, a variety of compounds such as anaromatic amine compound, a carbazole compound, an aromatic hydrocarbon,and a high molecular compound (such as an oligomer, a dendrimer, or apolymer) can be used. An organic compound having a hole mobility of1×10⁻⁶ cm²/Vs or higher is preferably used. However, any other substancemay be used as long as the substance has a hole-transport propertyhigher than an electron-transport property. The composite material of anorganic compound and a metal oxide has a high carrier-injection propertyand a high carrier-transport property; thus, low-voltage driving andlow-current driving can be achieved. Note that when a surface of alight-emitting unit on the anode side is in contact with a chargegeneration layer, the charge generation layer can also serve as ahole-transport layer of the light-emitting unit; thus, a hole-transportlayer does not need to be formed in the light-emitting unit.

The charge-generation layer 513 may have a stacked-layer structure of alayer containing the composite material of an organic compound and ametal oxide and a layer containing another material. For example, thecharge-generation layer 513 may have a stacked-layer structure of alayer containing the composite material of an organic compound and ametal oxide and a layer containing a compound of a substance selectedfrom electron-donating substances and a compound having a highelectron-transport property. Moreover, the charge-generation layer 513may have a stacked-layer structure of a layer containing the compositematerial of an organic compound and a metal oxide and a transparentconductive film.

The charge-generation layer 513 provided between the firstlight-emitting unit 511 and the second light-emitting unit 512 may haveany structure as far as electrons can be injected to a light-emittingunit on one side and holes can be injected to a light-emitting unit onthe other side when a voltage is applied between the first electrode 501and the second electrode 502. For example, in FIG. 2, any layer can beused as the charge generation layer 513 as far as the layer injectselectrons into the first light-emitting unit 511 and holes into thesecond light-emitting unit 512 when a voltage is applied such that thepotential of the first electrode is higher than that of the secondelectrode.

Although the light-emitting element having two light-emitting units isillustrated in FIG. 2, one embodiment of the present invention can besimilarly applied to a light-emitting element in which three or morelight-emitting units are stacked. With a plurality of light-emittingunits partitioned by the charge-generation layer between a pair ofelectrodes as in the light-emitting element of this embodiment, it ispossible to provide a light-emitting element that can emit light withhigh luminance with the current density kept low and has a longlifetime. Moreover, a light-emitting device having low driving voltageand lower power consumption can be achieved.

Further, when emission colors of the light-emitting units are madedifferent, light emission having a desired color can be obtained fromthe light-emitting element as a whole. For example, in thelight-emitting element having two light-emitting units, when the firstlight-emitting unit emits light of red and green and the secondlight-emitting unit emits light of blue, it is possible to obtain alight-emitting element from which white light is emitted from the wholelight-emitting element.

When the above-described structure of the light-emitting layer 113 isapplied to at least one of the plurality of units, the number ofmanufacturing steps of the unit can be reduced; thus, a multicolorlight-emitting element which is advantageous for practical applicationcan be provided.

The above-described structure can be combined with any of the structuresin this embodiment and the other embodiments.

Embodiment 4

In this embodiment, a light-emitting device manufactured using thelight-emitting element described in any of Embodiments 1 to 3 will bedescribed.

In this embodiment, a light-emitting device manufactured using thelight-emitting element described in any of Embodiments 1 to 3 will bedescribed with reference to FIGS. 3A and 3B. Note that FIG. 3A is a topview of the light-emitting device and FIG. 3B is a cross-sectional viewtaken along the lines A-B and C-D in FIG. 3A. This light-emitting deviceincludes a driver circuit portion (source line driver circuit) 601, apixel portion 602, and a driver circuit portion (gate line drivercircuit) 603, which are to control light emission of a light-emittingelement and illustrated with dotted lines. Reference numeral 604 denotesa sealing substrate; 605, a sealing material; and 607, a spacesurrounded by the sealing material 605.

Reference numeral 608 denotes a wiring for transmitting signals to beinput to the source line driver circuit 601 and the gate line drivercircuit 603 and receiving signals such as a video signal, a clocksignal, a start signal, and a reset signal from an FPC (flexible printedcircuit) 609 serving as an external input terminal. Although only theFPC is illustrated here, a printed wiring board (PWB) may be attached tothe FPC. The light-emitting device in the present specificationincludes, in its category, not only the light-emitting device itself butalso the light-emitting device provided with the FPC or the PWB.

Next, a cross-sectional structure is described with reference to FIG.3B. The driver circuit portion and the pixel portion are formed over anelement substrate 610; FIG. 3B shows the source line driver circuit 601,which is a driver circuit portion, and one of the pixels in the pixelportion 602.

As the source line driver circuit 601, a CMOS circuit in which ann-channel TFT 623 and a p-channel TFT 624 are combined is formed. Inaddition, the driver circuit may be formed with any of a variety ofcircuits such as a CMOS circuit, a PMOS circuit, or an NMOS circuit.Although a driver integrated type in which the driver circuit is formedover the substrate is illustrated in this embodiment, the driver circuitis not necessarily formed over the substrate, and the driver circuit canbe formed outside, not over the substrate.

The pixel portion 602 includes a plurality of pixels including aswitching TFT 611, a current controlling TFT 612, and a first electrode613 electrically connected to a drain of the current controlling TFT612. Note that to cover an end portion of the first electrode 613, aninsulator 614 is formed, for which a positive photosensitive acrylicresin film is used here.

In order to improve coverage, the insulator 614 is formed to have acurved surface with curvature at its upper or lower end portion. Forexample, in the case where positive photosensitive acrylic is used for amaterial of the insulator 614, only the upper end portion of theinsulator 614 preferably has a curved surface with a curvature radius(0.2 μm to 3 μm). As the insulator 614, either a negative photosensitiveresin or a positive photosensitive resin can be used.

An EL layer 616 and a second electrode 617 are formed over the firstelectrode 613. Here, as a material used for the first electrode 613functioning as an anode, a material having a high work function ispreferably used. For example, a single-layer film of an ITO film, anindium tin oxide film containing silicon, an indium oxide filmcontaining zinc oxide at 2 wt % to 20 wt %, a titanium nitride film, achromium film, a tungsten film, a Zn film, a Pt film, or the like, astack of a titanium nitride film and a film containing aluminum as itsmain component, a stack of three layers of a titanium nitride film, afilm containing aluminum as its main component, and a titanium nitridefilm, or the like can be used. The stacked-layer structure enables lowwiring resistance, favorable ohmic contact, and a function as an anode.

The EL layer 616 is formed by any of a variety of methods such as anevaporation method using an evaporation mask, an inkjet method, and aspin coating method. The EL layer 616 has the structure described in anyof Embodiments 1 to 3. Further, for another material included in the ELlayer 616, any of low molecular-weight compounds and polymeric compounds(including oligomers and dendrimers) may be used.

As a material used for the second electrode 617, which is formed overthe EL layer 616 and functions as a cathode, a material having a lowwork function (e.g., Al, Mg, Li, Ca, or an alloy or a compound thereof,such as MgAg, MgIn, or Al—Li) is preferably used. In the case wherelight generated in the EL layer 616 passes through the second electrode617, a stack of a thin metal film and a transparent conductive film(e.g., ITO, indium oxide containing zinc oxide at 2 wt % to 20 wt %,indium tin oxide containing silicon, or zinc oxide (ZnO)) is preferablyused for the second electrode 617.

Note that the light-emitting element is formed with the first electrode613, the EL layer 616, and the second electrode 617. The light-emittingelement has the structure described in any of Embodiments 1 to 3. In thelight-emitting device of this embodiment, the pixel portion, whichincludes a plurality of light-emitting elements, may include both thelight-emitting element described in any of Embodiments 1 to 3 and alight-emitting element having a different structure.

The sealing substrate 604 is attached to the element substrate 610 withthe sealing material 605, so that the light-emitting element 618 isprovided in the space 607 surrounded by the element substrate 610, thesealing substrate 604, and the sealing material 605. The space 607 maybe filled with filler, or may be filled with an inert gas (such asnitrogen or argon), or the sealing material 605. It is preferable thatthe sealing substrate be provided with a recessed portion and the dryingagent 625 be provided in the recessed portion, in which casedeterioration due to influence of moisture can be suppressed.

An epoxy-based resin or glass frit is preferably used for the sealingmaterial 605. It is preferable that such a material do not transmitmoisture or oxygen as much as possible. As the sealing substrate 604, aglass substrate, a quartz substrate, or a plastic substrate formed offiberglass reinforced plastic (FRP), polyvinyl fluoride (PVF),polyester, acrylic, or the like can be used.

As described above, the light-emitting device which uses thelight-emitting element described in any of Embodiments 1 to 3 can beobtained.

The light-emitting device in this embodiment is fabricated using thelight-emitting element described in any of Embodiments 1 to 3 and thuscan have favorable characteristics. Specifically, since thelight-emitting element described in any of Embodiments 1 to 3 has highemission efficiency, the light-emitting device can have reduced powerconsumption. In addition, since the light-emitting element can be drivenat low voltage, the light-emitting device can be driven at low voltage.

Although an active matrix light-emitting device is described above inthis embodiment, application to a passive matrix light-emitting devicemay be carried out. FIGS. 4A and 4B illustrate a passive matrixlight-emitting device manufactured using the present invention. FIG. 4Ais a perspective view of the light-emitting device, and FIG. 4B is across-sectional view taken along the line X-Y in FIG. 4A. In FIGS. 4Aand 4B, over a substrate 951, an EL layer 955 is provided between anelectrode 952 and an electrode 956. An end portion of the electrode 952is covered with an insulating layer 953. A partition layer 954 isprovided over the insulating layer 953. The sidewalls of the partitionlayer 954 are aslope such that the distance between both sidewalls isgradually narrowed toward the surface of the substrate. In other words,a cross section taken along the direction of the short side of thepartition layer 954 is trapezoidal, and the lower side (a side which isin the same direction as a plane direction of the insulating layer 953and in contact with the insulating layer 953) is shorter than the upperside (a side which is in the same direction as the plane direction ofthe insulating layer 953 and not in contact with the insulating layer953). The partition layer 954 thus provided can prevent defects in thelight-emitting element due to static electricity or the like. Thepassive matrix light-emitting device can also be driven with low powerconsumption by including the light-emitting element in any ofEmbodiments 1 to 3 which can be driven at low voltage. The passivematrix light-emitting device can also be driven with low powerconsumption by including the light-emitting element in any ofEmbodiments 1 to 3. Further, the passive matrix light-emitting devicecan have high reliability by including the light-emitting element in anyof Embodiments 1 to 3.

For performing full color display, a coloring layer or a colorconversion layer may be provided in a light path through which lightfrom the light-emitting element passes to the outside of thelight-emitting device. An example of a light-emitting device in whichfull color display is achieved with the use of a coloring layer and thelike is illustrated in FIGS. 5A and 5B. In FIG. 5A, a substrate 1001, abase insulating film 1002, a gate insulating film 1003, gate electrodes1006, 1007, and 1008, a first interlayer insulating film 1020, a secondinterlayer insulating film 1021, a peripheral portion 1042, a pixelportion 1040, a driver circuit portion 1041, first electrodes 1024W,1024R, 1024G, and 1024B of light-emitting elements, a partition 1025, anEL layer 1028, a second electrode 1029 of the light-emitting elements, asealing substrate 1031, a sealant 1032, and the like are illustrated.Further, coloring layers (a red coloring layer 1034R, a green coloringlayer 1034G, and a blue coloring layer 1034B) are provided on atransparent base material 1033. Further, a black layer (a black matrix)1035 may be additionally provided. The transparent base material 1033provided with the coloring layers and the black layer is positioned andfixed to the substrate 1001. Note that the coloring layers and the blacklayer are covered with an overcoat layer 1036. In this embodiment, lightemitted from part of the light-emitting layer does not pass through thecoloring layers, while light emitted from the other part of thelight-emitting layer passes through the coloring layers. Since lightwhich does not pass through the coloring layers is white and light whichpasses through any one of the coloring layers is red, blue, or green, animage can be displayed using pixels of the four colors.

The above-described light-emitting device is a light-emitting devicehaving a structure in which light is extracted from the substrate 1001side where the TFTs are formed (a bottom emission structure), but may bea light-emitting device having a structure in which light is extractedfrom the sealing substrate 1031 side (a top emission structure). FIG. 6is a cross-sectional view of a light-emitting device having a topemission structure. In this case, a substrate which does not transmitlight can be used as the substrate 1001. The process up to the step offorming of a connection electrode which connects the TFT and the anodeof the light-emitting element is performed in a manner similar to thatof the light-emitting device having a bottom emission structure. Then, athird interlayer insulating film 1037 is formed to cover an electrode1022. This insulating film may have a planarization function. The thirdinterlayer insulating film 1037 can be formed using a material similarto that of the second interlayer insulating film, and can alternativelybe formed using any other known material.

The first electrodes 1024W, 1024R, 1024G, and 1024B of thelight-emitting elements each serve as an anode here, but may serve as acathode. Further, in the case of a light-emitting device having a topemission structure as illustrated in FIG. 6, the first electrodes arepreferably reflective electrodes. The EL layer 1028 is formed to have astructure similar to the structure described in any of Embodiments 1 to3, with which white light emission can be obtained. As the structurewith which white light emission can be obtained, in the case where twoEL layers are used, a structure with which blue light is obtained from alight-emitting layer in one of the EL layers and orange light isobtained from a light-emitting layer of the other of the EL layers; astructure in which blue light is obtained from a light-emitting layer ofone of the EL layers and red light and green light are obtained from alight-emitting layer of the other of the EL layers; and the like can begiven. Further, in the case where three EL layers are used, red light,green light, and blue light are obtained from respective light-emittinglayers, so that a light-emitting element which emits white light can beobtained. Needless to say, the structure with which white light emissionis obtained is not limited thereto as long as the structure described inany of Embodiments 1 to 3 is used.

The coloring layers are each provided in a light path through whichlight from the light-emitting element passes to the outside of thelight-emitting device. In the case of the light-emitting device having abottom emission structure as illustrated in FIG. 5A, the coloring layers1034R, 1034G, and 1034B can be provided on the transparent base material1033 and then fixed to the substrate 1001. The coloring layers may beprovided between the gate insulating film 1003 and the first interlayerinsulating film 1020 as illustrated in FIG. 5B. In the case of a topemission structure as illustrated in FIG. 6, sealing can be performedwith the sealing substrate 1031 on which the coloring layers (the redcoloring layer 1034R, the green coloring layer 1034G, and the bluecoloring layer 1034B) are provided. The sealing substrate 1031 may beprovided with the black layer (the black matrix) 1035 which ispositioned between pixels. The coloring layers (the red coloring layer1034R, the green coloring layer 1034G and the blue coloring layer 1034B)and the black layer (the black matrix) may be covered with the overcoatlayer 1036. Note that a light-transmitting substrate is used as thesealing substrate 1031.

When voltage is applied between the pair of electrodes of the thusobtained organic light-emitting element, a white light-emitting region1044W can be obtained. In addition, by using the coloring layers, a redlight-emitting region 1044R, a blue light-emitting region 1044B, and agreen light-emitting region 1044G can be obtained. The light-emittingdevice in this embodiment includes the light-emitting element describedin any of Embodiments 1 to 3; thus, a light-emitting device with lowpower consumption can be obtained.

Although an example in which full color display is performed using fourcolors of red, green, blue, and white is shown here, there is noparticular limitation and full color display using three colors of red,green, and blue may be performed.

This embodiment can be freely combined with any of the otherembodiments.

Embodiment 5

In this embodiment, an example in which the light-emitting elementdescribed in any of Embodiments 1 to 3 is used for a lighting devicewill be described with reference to FIGS. 7A and 7B. FIG. 7B is a topview of the lighting device, and FIG. 7A is a cross-sectional view takenalong the line e-f in FIG. 7B.

In the lighting device in this embodiment, a first electrode 401 isformed over a substrate 400 which is a support and has alight-transmitting property. The first electrode 401 corresponds to thefirst electrode 101 in Embodiments 1 to 3.

An auxiliary electrode 402 is provided over the first electrode 401.Since light emission is extracted through the first electrode 401 sidein the example given in this embodiment, the first electrode 401 isformed using a material having a light-transmitting property. Theauxiliary electrode 402 is provided in order to compensate for the lowconductivity of the material having a light-transmitting property, andhas a function of suppressing luminance unevenness in a light emissionsurface due to voltage drop caused by the high resistance of the firstelectrode 401. The auxiliary electrode 402 is formed using a materialhaving at least higher conductivity than the material of the firstelectrode 401, and is preferably formed using a material having highconductivity such as aluminum. Note that surfaces of the auxiliaryelectrode 402 other than a portion thereof in contact with the firstelectrode 401 are preferably covered with an insulating layer. This isfor suppressing light emission over the upper portion of the auxiliaryelectrode 402, which cannot be extracted, and for suppressing areduction in power efficiency. Note that a pad 412 for applying avoltage to a second electrode 404 may be formed at the same time as theauxiliary electrode 402.

An EL layer 403 is formed over the first electrode 401 and the auxiliaryelectrode 402. The EL layer 403 has the structure described in any ofEmbodiments 1 to 3. Refer to the descriptions for the structure. Notethat the EL layer 403 is preferably formed to be slightly larger thanthe first electrode 401 when seen from above, in which case the EL layer403 can also serve as an insulating layer that suppresses a shortcircuit between the first electrode 401 and the second electrode 404.

The second electrode 404 is formed to cover the EL layer 403. The secondelectrode 404 corresponds to the second electrode 102 in Embodiments 1to 3 and has a similar structure. In this embodiment, it is preferablethat the second electrode 404 be formed using a material having highreflectance because light emission is extracted through the firstelectrode 401 side. In this embodiment, the second electrode 404 isconnected to the pad 412, whereby voltage is applied.

As described above, the lighting device described in this embodimentincludes a light-emitting element including the first electrode 401, theEL layer 403, and the second electrode 404 (and the auxiliary electrode402). Since the light-emitting element is a light-emitting element withhigh emission efficiency, the lighting device in this embodiment can bea lighting device having low power consumption. Furthermore, since thelight-emitting element has high reliability, the lighting device in thisembodiment can be a lighting device having high reliability.

The light-emitting element having the above structure is fixed to asealing substrate 407 with sealing materials 405 and sealing isperformed, whereby the lighting device is completed. It is possible touse only one of the sealing materials 405. The inner sealing material405 can be mixed with a desiccant which enables moisture to be adsorbed,increasing reliability.

When parts of the pad 412, the first electrode 401, and the auxiliaryelectrode 402 are extended to the outside of the sealing materials 405and 406, the extended parts can serve as external input terminals. An ICchip 420 mounted with a converter or the like may be provided over theexternal input terminals.

As described above, since the lighting device described in thisembodiment includes the light-emitting element described in any ofEmbodiments 1 to 3 as an EL element, the lighting device can be alighting device having low power consumption. Further, the lightingdevice can be a lighting device which can be driven at low voltage.Furthermore, the lighting device can be a lighting device having highreliability.

Embodiment 6

In this embodiment, examples of electronic devices each including thelight-emitting element described in any of Embodiments 1 to 3 will bedescribed. The light-emitting element described in any of Embodiments 1to 3 has high emission efficiency and reduced power consumption. As aresult, the electronic devices described in this embodiment can eachinclude a light-emitting portion having reduced power consumption. Inaddition, the electronic devices can be driven at low voltage since thelight-emitting element described in any of Embodiments 1 to 3 can bedriven at low voltage.

Examples of the electronic device to which the above light-emittingelement is applied include television devices (also referred to as TV ortelevision receivers), monitors for computers and the like, cameras suchas digital cameras and digital video cameras, digital photo frames,mobile phones (also referred to as cell phones or mobile phone devices),portable game machines, portable information terminals, audio playbackdevices, large game machines such as pachinko machines, and the like.Specific examples of these electronic devices are described below.

FIG. 8A illustrates an example of a television device. In the televisiondevice, a display portion 7103 is incorporated in a housing 7101. Here,the housing 7101 is supported by a stand 7105. Images can be displayedon the display portion 7103, and in the display portion 7103, thelight-emitting elements described in any of Embodiments 1 to 3 arearranged in a matrix. The light-emitting elements can have high emissionefficiency. Further, the light-emitting elements can be driven at lowvoltage. Furthermore, the light-emitting elements can have a longlifetime. Therefore, the television device including the display portion7103 which is formed using the light-emitting elements can exhibitreduced power consumption. Further, the television device can be drivenat low voltage. Furthermore, the television device can have highreliability.

Operation of the television device can be performed with an operationswitch of the housing 7101 or a separate remote controller 7110. Withoperation keys 7109 of the remote controller 7110, channels and volumecan be controlled and images displayed on the display portion 7103 canbe controlled. The remote controller 7110 may be provided with a displayportion 7107 for displaying data output from the remote controller 7110.

Note that the television device is provided with a receiver, a modem,and the like. With the use of the receiver, general televisionbroadcasting can be received. Moreover, when the television device isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

FIG. 8B1 illustrates a computer, which includes a main body 7201, ahousing 7202, a display portion 7203, a keyboard 7204, an externalconnection port 7205, a pointing device 7206, and the like. Note thatthis computer is manufactured by arranging light-emitting elementssimilar to those described in any of Embodiments 1 to 3 in a matrix inthe display portion 7203. The computer illustrated in FIG. 8B1 may havea structure illustrated in FIG. 8B2. The computer illustrated in FIG.8B2 is provided with a second display portion 7210 instead of thekeyboard 7204 and the pointing device 7206. The second display portion7210 has a touch screen, and input can be performed by operation ofimages, which are displayed on the second display portion 7210, with afinger or a dedicated pen. The second display portion 7210 can alsodisplay images other than the display for input. The display portion7203 may also have a touch screen. Connecting the two screens with ahinge can prevent troubles; for example, the screens can be preventedfrom being cracked or broken while the computer is being stored orcarried. Note that this computer is manufactured by arranging thelight-emitting elements described in any of Embodiments 1 to 3 in amatrix in the display portion 7203. The light-emitting elements can havehigh emission efficiency. Therefore, this computer having the displayportion 7203 which is formed using the light-emitting elements can havereduced power consumption.

FIG. 8C illustrates a portable game machine having two housings, ahousing 7301 and a housing 7302, which are connected with a jointportion 7303 so that the portable game machine can be opened or folded.The housing 7301 incorporates a display portion 7304 including thelight-emitting elements described in any of Embodiments 1 to 3 andarranged in a matrix, and the housing 7302 incorporates a displayportion 7305. In addition, the portable game machine illustrated in FIG.8C includes a speaker portion 7306, a recording medium insertion portion7307, an LED lamp 7308, input means (an operation key 7309, a connectionterminal 7310, a sensor 7311 (a sensor having a function of measuringforce, displacement, position, speed, acceleration, angular velocity,rotational frequency, distance, light, liquid, magnetism, temperature,chemical substance, sound, time, hardness, electric field, current,voltage, electric power, radiation, flow rate, humidity, gradient,oscillation, odor, or infrared rays), and a microphone 7312), and thelike. Needless to say, the structure of the portable game machine is notlimited to the above as long as the display portion which includes thelight-emitting elements described in any of Embodiments 1 to 3 andarranged in a matrix is used as at least either the display portion 7304or the display portion 7305, or both, and the structure can includeother accessories as appropriate. The portable game machine illustratedin FIG. 8C has a function of reading out a program or data stored in astorage medium to display it on the display portion, and a function ofsharing information with another portable game machine by wirelesscommunication. Note that functions of the portable game machineillustrated in FIG. 8C are not limited to them, and the portable gamemachine can have various functions. Since the light-emitting elementsused in the display portion 7304 have high emission efficiency, theportable game machine including the above-described display portion 7304can have reduced power consumption. Since each of the light-emittingelements used in the display portion 7304 can be driven at low voltage,the portable game machine can also be driven at low voltage.Furthermore, since the light-emitting elements used in the displayportion 7304 each have a long lifetime, the portable game machine canhave high reliability.

FIG. 8D illustrates an example of a mobile phone. The mobile phone isprovided with a display portion 7402 incorporated in a housing 7401,operation buttons 7403, an external connection port 7404, a speaker7405, a microphone 7406, and the like. Note that the mobile phone hasthe display portion 7402 including the light-emitting elements describedin any of Embodiments 1 to 3 and arranged in a matrix. Thelight-emitting elements can have high emission efficiency. Further, thelight-emitting elements can be driven at low voltage. Furthermore, thelight-emitting elements can have a long lifetime. Therefore, the mobilephone including the display portion 7402 which is found using thelight-emitting elements can have reduced power consumption. Further, themobile phone can be driven at low voltage. Furthermore, the mobile phonecan have high reliability.

When the display portion 7402 of the mobile phone illustrated in FIG. 8Dis touched with a finger or the like, data can be input into the mobilephone. In this case, operations such as making a call and creating ane-mail can be performed by touching the display portion 7402 with afinger or the like.

There are mainly three screen modes of the display portion 7402. Thefirst mode is a display mode mainly for displaying an image. The secondmode is an input mode mainly for inputting information such ascharacters. The third mode is a display-and-input mode in which twomodes of the display mode and the input mode are combined.

For example, in the case of making a call or creating an e-mail, acharacter input mode mainly for inputting characters is selected for thedisplay portion 7402 so that characters displayed on a screen can beinput. In this case, it is preferable to display a keyboard or numberbuttons on almost the entire screen of the display portion 7402.

When a detection device including a sensor for detecting inclination,such as a gyroscope or an acceleration sensor, is provided inside themobile phone, display on the screen of the display portion 7402 can beautomatically changed by determining the orientation of the mobile phone(whether the mobile phone is placed horizontally or vertically for alandscape mode or a portrait mode).

The screen modes are switched by touch on the display portion 7402 oroperation with the operation buttons 7403 of the housing 7401. Thescreen modes can be switched depending on the kinds of images displayedon the display portion 7402. For example, when a signal of an imagedisplayed on the display portion is a signal of moving image data, thescreen mode is switched to the display mode. When the signal is a signalof text data, the screen mode is switched to the input mode.

Moreover, in the input mode, when input by touching the display portion7402 is not performed for a certain period while a signal detected by anoptical sensor in the display portion 7402 is detected, the screen modemay be controlled so as to be switched from the input mode to thedisplay mode.

The display portion 7402 may function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken by thedisplay portion 7402 while in touch with the palm or the finger, wherebypersonal authentication can be performed. Further, by providing abacklight or a sensing light source which emits a near-infrared light inthe display portion, an image of a finger vein, a palm vein, or the likecan be taken.

Note that the structure described in this embodiment can be combinedwith any of the structures described in Embodiments 1 to 5 asappropriate.

As described above, the application range of the light-emitting devicehaving the light-emitting element described in any of Embodiments 1 to 3is wide so that this light-emitting device can be applied to electronicdevices in a variety of fields. By using the light-emitting elementdescribed in any of Embodiments 1 to 3, an electronic device havingreduced power consumption can be obtained.

FIG. 9 illustrates an example of a liquid crystal display device usingthe light-emitting element described in any of Embodiments 1 to 3 for abacklight. The liquid crystal display device illustrated in FIG. 9includes a housing 901, a liquid crystal layer 902, a backlight unit903, and a housing 904. The liquid crystal layer 902 is connected to adriver IC 905. The light-emitting element described in any ofEmbodiments 1 to 3 is used in the backlight unit 903, to which currentis supplied through a terminal 906.

The light-emitting element described in any of Embodiments 1 to 3 isused for the backlight of the liquid crystal display device; thus, thebacklight can have reduced power consumption. In addition, the use ofthe light-emitting element described in any of Embodiments 1 to 3enables manufacture of a planar-emission lighting device and further alarger-area planar-emission lighting device; therefore, the backlightcan be a larger-area backlight, and the liquid crystal display devicecan also be a larger-area device. Furthermore, the light-emitting deviceusing the light-emitting element described in any of Embodiments 1 to 3can be thinner than a conventional one; accordingly, the display devicecan also be thinner.

FIG. 10 illustrates an example in which the light-emitting elementdescribed in any of Embodiments 1 to 3 is used for a table lamp which isa lighting device. The table lamp illustrated in FIG. 10 includes ahousing 2001 and a light source 2002, and the light-emitting elementdescribed in any of Embodiments 1 to 3 is used for the light source2002.

FIG. 11 illustrates an example in which the light-emitting elementdescribed in any of Embodiments 1 to 3 is used for an indoor lightingdevice 3001. Since the light-emitting element described in any ofEmbodiments 1 to 3 has reduced power consumption, a lighting device thathas reduced power consumption can be obtained. Further, since thelight-emitting element described in any of Embodiments 1 to 3 can have alarge area, the light-emitting element can be used for a large-arealighting device. Furthermore, since the light-emitting element describedin any of Embodiments 1 to 3 is thin, the light-emitting element can beused for a lighting device having a reduced thickness.

The light-emitting element described in any of Embodiments 1 to 3 canalso be used for an automobile windshield or an automobile dashboard.FIG. 12 illustrates one mode in which the light-emitting elementsdescribed in any of Embodiments 1 to 3 are used for an automobilewindshield and an automobile dashboard. Displays regions 5000 to 5005each include the light-emitting element described in any of Embodiments1 to 3.

The display regions 5000 and the display region 5001 are provided in theautomobile windshield in which the light-emitting elements described inany of Embodiments 1 to 3 are incorporated. The light-emitting elementdescribed in any of Embodiments 1 to 3 can be formed into what is calleda see-through display device, through which the opposite side can beseen, by including a first electrode and a second electrode formed ofelectrodes having light-transmitting properties. Such see-throughdisplay devices can be provided even in the automobile windshield,without hindering the vision. Note that in the case where a transistorfor driving or the like is provided, a transistor having alight-transmitting property, such as an organic transistor using anorganic semiconductor material or a transistor using an oxidesemiconductor, is preferably used.

The display region 5002 is provided in a pillar portion in which thelight-emitting elements described in any of Embodiments 1 to 3 areincorporated. The display region 5002 can compensate for the viewhindered by the pillar portion by showing an image taken by an imagingunit provided in the car body. Similarly, the display region 5003provided in the dashboard can compensate for the view hindered by thecar body by showing an image taken by an imaging unit provided in theoutside of the car body, which leads to elimination of blind areas andenhancement of safety. Showing an image so as to compensate for the areawhich a driver cannot see makes it possible for the driver to confirmsafety easily and comfortably.

The display region 5004 and the display region 5005 can provide avariety of kinds of information such as navigation data, a speedometer,a tachometer, a mileage, a fuel meter, a gearshift indicator, andair-condition setting. The content or layout of the display can bechanged freely by a user as appropriate. Note that such information canalso be shown by the display regions 5000 to 5003. The display regions5000 to 5005 can also be used as lighting devices.

The light-emitting element described in any of Embodiments 1 to 3 canhave high emission efficiency and low power consumption. Therefore, loadon a battery is small even when a number of large screens such as thedisplay regions 5000 to 5005 are provided, which provides comfortableuse. For that reason, the light-emitting device and the lighting deviceeach of which includes the light-emitting element described in any ofEmbodiments 1 to 3 can be suitably used as an in-vehicle light-emittingdevice and an in-vehicle lighting device.

FIGS. 13A and 13B illustrate an example of a foldable tablet terminal.FIG. 13A illustrates the tablet terminal which is unfolded. The tabletterminal includes a housing 9630, a display portion 9631 a, a displayportion 9631 b, a display mode switch 9034, a power switch 9035, apower-saving mode switch 9036, a clasp 9033, and an operation switch9038. Note that in the tablet terminal, one of or both the displayportion 9631 a and the display portion 9631 b is/are formed using alight-emitting device which includes the light-emitting elementdescribed in any of Embodiments 1 to 3.

Part of the display portion 9631 a can be a touchscreen region 9632 aand data can be input when a displayed operation key 9637 is touched.Although half of the display portion 9631 a has only a display functionand the other half has a touchscreen function, one embodiment of thepresent invention is not limited to the structure. The whole displayportion 9631 a may have a touchscreen function. For example, a keyboardcan be displayed on the entire region of the display portion 9631 a sothat the display portion 9631 a is used as a touchscreen, and thedisplay portion 9631 b can be used as a display screen.

Like the display portion 9631 a, part of the display portion 9631 b canbe a touchscreen region 9632 b. When a switching button 9639 forshowing/hiding a keyboard on the touchscreen is touched with a finger, astylus, or the like, the keyboard can be displayed on the displayportion 9631 b.

Touch input can be performed in the touchscreen region 9632 a and thetouchscreen region 9632 b at the same time.

The display mode switch 9034 can switch the display between portraitmode, landscape mode, and the like, and between monochrome display andcolor display, for example. With the switch 9036 for switching topower-saving mode, the luminance of display can be optimized inaccordance with the amount of external light at the time when the tabletterminal is in use, which is detected with an optical sensorincorporated in the tablet terminal. The tablet terminal may includeanother detection device such as a sensor for detecting orientation(e.g., a gyroscope or an acceleration sensor) in addition to the opticalsensor.

Although FIG. 13A illustrates an example in which the display portion9631 a and the display portion 9631 b have the same display area, oneembodiment of the present invention is not limited to the example. Thedisplay portion 9631 a and the display portion 9631 b may have differentdisplay areas and different display quality. For example, higherdefinition images may be displayed on one of the display portions 9631 aand 9631 b.

FIG. 13B illustrates the tablet terminal which is folded. The tabletterminal in this embodiment includes the housing 9630, a solar cell9633, a charge and discharge control circuit 9634, a battery 9635, and aDC-to-DC converter 9636. Note that FIG. 13B illustrates an example inwhich the charge and discharge control circuit 9634 includes the battery9635 and the DC-to-DC converter 9636.

Since the tablet terminal is foldable, the housing 9630 can be closedwhen the tablet terminal is not in use. As a result, the display portion9631 a and the display portion 9631 b can be protected, therebyproviding a tablet terminal with high endurance and high reliability forlong-term use.

The tablet terminal illustrated in FIGS. 13A and 13B can have otherfunctions such as a function of displaying various kinds of data (e.g.,a still image, a moving image, and a text image), a function ofdisplaying a calendar, a date, the time, or the like on the displayportion, a touch-input function of operating or editing the datadisplayed on the display portion by touch input, and a function ofcontrolling processing by various kinds of software (programs).

The solar cell 9633 provided on a surface of the tablet terminal cansupply power to the touchscreen, the display portion, a video signalprocessing portion, or the like. Note that the solar cell 9633 ispreferably provided on one or two surfaces of the housing 9630, in whichcase the battery 9635 can be charged efficiently.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 13B will be described with reference to a blockdiagram of FIG. 13C. FIG. 13C illustrates the solar cell 9633, thebattery 9635, the DC-to-DC converter 9636, a converter 9638, switchesSW1 to SW3, and a display portion 9631. The battery 9635, the DC-to-DCconverter 9636, the converter 9638, and the switches SW1 to SW3correspond to the charge and discharge control circuit 9634 illustratedin FIG. 13B.

First, description is made on an example of the operation in the casewhere power is generated by the solar cell 9633 with the use of externallight. The voltage of the power generated by the solar cell is raised orlowered by the DC-to-DC converter 9636 so as to be voltage for chargingthe battery 9635. Then, when power from the solar cell 9633 is used forthe operation of the display portion 9631, the switch SW1 is turned onand the voltage of the power is raised or lowered by the converter 9638so as to be voltage needed for the display portion 9631. When images arenot displayed on the display portion 9631, the switch SW1 is turned offand the switch SW2 is turned on so that the battery 9635 is charged.

Although the solar cell 9633 is described as an example of a powergeneration means, the power generation means is not particularlylimited, and the battery 9635 may be charged by another power generationmeans such as a piezoelectric element or a thermoelectric conversionelement (Peltier element). The battery 9635 may be charged by anon-contact power transmission module capable of performing charging bytransmitting and receiving power wirelessly (without contact), or any ofthe other charge means used in combination, and the power generationmeans is not necessarily provided.

One embodiment of the present invention is not limited to the tabletterminal having the shape illustrated in FIGS. 13A to 13C as long as thedisplay portion 9631 a or 9631 b is included.

Example 1

In this example, a method for fabricating a light-emitting element whichcorresponds to one embodiment of the present invention described in anyof Embodiments 1 to 3 and the characteristics thereof will be described.Structural formulae of organic compounds used in this example are shownbelow.

Next, a method for fabricating the light-emitting element in thisexample will be described below.

First, a film of indium oxide-tin oxide containing silicon oxide (ITSO)was formed over a glass substrate by a sputtering method, so that thefirst electrode 101 was formed. The thickness thereof was 110 nm and theelectrode area was 2 mm×2 mm. Here, the first electrode 101 functions asan anode of the light-emitting element.

Next, as pretreatment for forming the light-emitting element over thesubstrate, UV-ozone treatment was performed for 370 seconds afterwashing of a surface of the substrate with water and baking that wasperformed at 200° C. for one hour.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate was cooled down for about 30 minutes.

Then, the substrate over which the first electrode 101 was formed wasfixed to a substrate holder provided in the vacuum evaporation apparatusso that the surface on which the first electrode 101 was formed faceddownward. The pressure in the vacuum evaporation apparatus was reducedto about 10⁻⁴ Pa. After that, over the first electrode 101,4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II) represented by Structural Formula (i) and molybdenum(VI) oxidewere deposited by co-evaporation, so that the hole-injection layer 111was formed. The thickness of the hole-injection layer 111 was set to 40nm, and the weight ratio of DBT3P-II to molybdenum oxide was adjusted to4:2. Note that the co-evaporation method refers to an evaporation methodin which evaporation is carried out from a plurality of evaporationsources at the same time in one treatment chamber.

Next, a film of 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP) which is represented by Structural Formula (ii)was formed to a thickness of 20 nm over the hole-injection layer 111 toform the hole-transport layer 112.

Over the hole-transport layer 112,2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II) represented by Structural Formula (iii),4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBA1BP) represented by Structural Formula (iv), and(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III)(abbreviation: [Ir(tppr)₂(dpm)]) represented by Structural Formula (v)were deposited by co-evaporation to a thickness of 20 nm with a weightratio of 2mDBTPDBq-II to PCBA1BP and [Ir(tppr)₂(dpm)] being0.5:0.5:0.05, so that the second light-emitting layer 113 b was formed;after that, 3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (abbreviation:35DCzPPy) represented by Structural Formula (vi),3,3′-bis(9-phenyl-9H-carbazole) (abbreviation: PCCP) represented byStructural Formula (vii), andtris{2-[5-(2-methylphenyl)-4-(2,6-dimethylphenyl)-4H-1,2,4-triazol-3-yl-κN2]phenyl-κC}iridium(III)(abbreviation: [Ir(mpptz-dmp)₃]) represented by Structural Formula(viii) (the compound (1)) were deposited by co-evaporation to athickness of 30 nm with a weight ratio of 35DCzPPy to PCCP and[Ir(mpptz-dmp)₃] being 0.5:0.5:0.06, so that the first light-emittinglayer 113 a was formed.

Then, the electron-transport layer 114 was formed over thelight-emitting layer 113 in such a way that a 10-nm-thick film of2-[3-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole(abbreviation: mDBTBIm-II) represented by Structural formula (ix) wasformed and a 20-nm-thick film of bathophenanthroline (abbreviation:BPhen) represented by Structural Formula (x) was formed.

After the formation of the electron-transport layer 114, lithiumfluoride (LiF) was deposited by evaporation to a thickness of 1 nm, sothat the electron-injection layer 115 was formed.

Lastly, aluminum was deposited by evaporation to a thickness of 200 nmto form the second electrode 102 functioning as a cathode. Thus, alight-emitting element 1 in this example was fabricated.

Note that in all the above evaporation steps, evaporation was performedby a resistance-heating method.

Table 1 shows an element structure of the light-emitting element 1obtained in the above manner.

TABLE 1 Hole- Hole- Electron- First injection transport Light- Electron-injection Second electrode layer layer emitting layer transport layerlayer electrode Light-emitting ITSO DBT3P-II:MoOx BPAFLP * ** mDBTBIm-IIBphen LiF Al element 1 (110 nm) (4:2 40 nm) (20 nm) (10 nm) (20 nm) (1nm) (200 nm) * 2mDBTPDBq-II:PCBA1BP:[Ir(tppr)₂(dpm)] (0.5:0.5:0.05 20nm) ** 35DCzPPy:PCCP:[Ir(mpptz-dmp)₃] (0.5:0.5:0.06 30 nm)

The light-emitting element 1 was sealed using a glass substrate in aglove box containing a nitrogen atmosphere so as not to be exposed tothe air (specifically, a sealing material was applied onto an outer edgeof the element and heat treatment was performed at 80° C. for 1 hour atthe time of sealing).

Element characteristics of the light-emitting element were measured.Note that the measurements were carried out at room temperature (in anatmosphere kept at 25° C.).

FIG. 14 shows an emission spectrum of the light-emitting element 1 whichwas obtained when a current of 0.1 mA was made to flow in thelight-emitting element 1. FIG. 14 indicates that the light-emittingelement 1 shows an emission spectrum including light with a wavelengthin the blue wavelength range which originates from [Ir(mpptz-dmp)₃] andlight with a wavelength in the red wavelength range which originatesfrom [Ir(tppr)₂(dpm)].

FIG. 15 shows luminance-current efficiency characteristics of thelight-emitting element 1; FIG. 16 shows luminance-external quantumefficiency characteristics thereof; FIG. 17 shows voltage-luminancecharacteristics thereof; and FIG. 18 shows luminance-power efficiencycharacteristics thereof. Table 2 shows main characteristics of thelight-emitting element 1 at around 1000 cd/m².

TABLE 2 Current Current Power External Voltage Current densityChromaticity efficiency efficiency quantum (V) (mA) (mA/cm²) (x, y)(cd/A) (lm/W) efficiency (%) Light-emitting 4.6 0.11 2.6 (0.53, 0.36) 3624 25 element 1

From the above, the light-emitting element 1 turned out to haveexcellent element characteristics. In particular, as can be seen fromFIG. 15 and FIG. 16, the light-emitting element 1 has extremely highemission efficiency and has a high external quantum efficiency of 25% ataround a practical luminance (1000 cd/m²). Further, FIG. 17 shows thatthe light-emitting element 1 has favorable voltage-luminancecharacteristics, and is driven at a low voltage. Thus, as is clear fromFIG. 18, the light-emitting element 1 has favorable power efficiency.

The above shows that the light-emitting element 1 corresponding to oneembodiment of the present invention has excellent elementcharacteristics and provides lights from two kinds of emission centersubstances in a good balance.

Example 2

In this example, a method for fabricating a light-emitting element whichcorresponds to one embodiment of the present invention described inEmbodiments 1 to 3 and the characteristics thereof will be described.Structural formulae of organic compounds used in this example are shownbelow. In this example, a light-emitting element 2 and a light-emittingelement 3 in each of which the light-emitting layer 113 includes thefirst light-emitting layer 113 a, the second light-emitting layer 113 b,and the third light-emitting layer 113 c were fabricated.

Next, a method for fabricating the light-emitting element 2 in thisexample will be described below.

First, a film of indium oxide-tin oxide containing silicon oxide (ITSO)was formed over a glass substrate by a sputtering method, so that thefirst electrode 101 was formed. The thickness thereof was 110 nm and theelectrode area was 2 mm×2 mm. Here, the first electrode 101 functions asan anode of the light-emitting element.

Next, as pretreatment for forming the light-emitting element over thesubstrate, UV-ozone treatment was performed for 370 seconds afterwashing of a surface of the substrate with water and baking that wasperformed at 200° C. for one hour.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate was cooled down for about 30 minutes.

Then, the substrate over which the first electrode 101 was formed wasfixed to a substrate holder provided in the vacuum evaporation apparatusso that the surface on which the first electrode 101 was formed faceddownward. The pressure in the vacuum evaporation apparatus was reducedto about 10⁻⁴ Pa. After that, over the first electrode 101, DBT3P-II andmolybdenum(VI) oxide were deposited by co-evaporation by an evaporationmethod using resistance heating, so that the hole-injection layer 111was formed. The thickness of the hole-injection layer 111 was set to 40nm, and the weight ratio of DBT3P-II to molybdenum oxide was adjusted to4:2. Note that the co-evaporation method refers to an evaporation methodin which evaporation is carried out from a plurality of evaporationsources at the same time in one treatment chamber.

Next, a film of BPAFLP was formed to a thickness of 20 nm over thehole-injection layer 111 to form the hole-transport layer 112.

Over the hole-transport layer 112, 2mDBTPDBq-II, PCBA1BP, and[Ir(tppr)₂(dpm)] were deposited by co-evaporation to a thickness of 10nm with a weight ratio of 2mDBTPDBq-II to PCBA1BP and [Ir(tppr)₂(dpm)]being 0.5:0.5:0.05, so that the third light-emitting layer 113 c wasformed; then, 2mDBTPDBq-II, PCBA1BP, and(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)(abbreviation: [Ir(tBuppm)₂(acac)]) represented by Structural Formula(xi) were deposited by co-evaporation to a thickness of 5 nm with aweight ratio of 2mDBTPDBq-II to PCBA1BP and [Ir(tBuppm)₂(acac)] being0.5:0.5:0.05, so that the second light-emitting layer 113 b was formed;after that, 35DCzPPy, PCCP, and [Ir(mpptz-dmp)₃] were deposited byco-evaporation to a thickness of 30 nm with a weight ratio of 35DCzPPyto PCCP and [Ir(mpptz-dmp)₃] being 0.5:0.5:0.06, so that the firstlight-emitting layer 113 a was faulted.

Then, the electron-transport layer 114 was formed over thelight-emitting layer 113 in such a way that a 10-nm-thick film ofmDBTBIm-II was formed and a 20-nm-thick film of BPhen was formed.

After the formation of the electron-transport layer 114, lithiumfluoride (LiF) was deposited by evaporation to a thickness of 1 nm, sothat the electron-injection layer 115 was formed.

Lastly, aluminum was deposited by evaporation to a thickness of 200 nmto form the second electrode 102 functioning as a cathode. Thus, thelight-emitting element 2 in this example was fabricated.

Note that in all the above evaporation steps, evaporation was performedby a resistance-heating method.

Next, a method for fabricating the light-emitting element 3 will bedescribed. The light-emitting element 3 was fabricated in such a mannerthat the thickness of the second light-emitting layer 113 b in thelight-emitting element 2 was changed from 5 nm to 10 nm. The otherstructures are the same as those of the light-emitting element 2.

Table 3 shows element structures of the light-emitting element 2 and thelight-emitting element 3 obtained in the above manner.

TABLE 3 Hole- Hole- Electron- First injection transport Light- Electron-injection Second electrode layer layer emitting layer transport layerlayer electrode Light-emitting ITSO DBT3P-II:MoOx BPAFLP * ** ***mDBTBIm-II Bphen LiF Al element 2 (110 nm) (4:2 40 nm) (20 nm) (10 nm)(20 nm) (1 nm) (200 nm) Light-emitting ITSO DBT3P-II:MoOx BPAFLP * ******* mDBTBIm-II Bphen LiF Al element 3 (110 nm) (4:2 40 nm) (20 nm) (10nm) (20 nm) (1 nm) (200 nm) * 2mDBTPDBq-II:PCBA1BP:[Ir(tppr)₂(dpm)](0.5:0.5:0.05 10 nm) ** 2mDBTPDBq-II:PCBA1BP:[Ir(tBuppm)₂(acac)](0.5:0.5:0.05 5 nm) *** 35DCzPPy:PCCP:[Ir(mpptz-dmp)₃] (0.5:0.5:0.06 30nm) **** 2mDBTPDBq-II:PCBA1BP:[Ir(tBuppm)₂(acac)] (0.5:0.5:0.05 10 nm)

The light-emitting element 2 and the light-emitting element 3 weresealed using a glass substrate in a glove box containing a nitrogenatmosphere so as not to be exposed to the air (specifically, a sealingmaterial was applied onto an outer edge of the element and heattreatment was performed at 80° C. for 1 hour at the time of sealing).

Element characteristics of the light-emitting elements were measured.Note that the measurements were carried out at room temperature (in anatmosphere kept at 25° C.).

FIG. 19 shows emission spectra of the light-emitting element 2 and thelight-emitting element 3 which were obtained when a current of 0.1 mAwas made to flow in the light-emitting element 2 and the light-emittingelement 3. FIG. 19 indicates that the light-emitting element 2 and thelight-emitting element 3 show emission spectra each including light witha wavelength in the blue wavelength range which originates from[Ir(mpptz-dmp)₃], light with a wavelength in the green wavelength rangewhich originates from [Ir(tBuppm)₂(acac)], and light with a wavelengthin the red wavelength range which originates from [Ir(tppr)₂(dpm)]. Inparticular, the light-emitting element 2 emits light that meets thestandards defined by JIS.

FIG. 20 shows luminance-current efficiency characteristics of thelight-emitting element 2 and the light-emitting element 3; FIG. 21 showsluminance-external quantum efficiency characteristics thereof; FIG. 22shows voltage-luminance characteristics thereof; and FIG. 23 showsluminance-power efficiency characteristics thereof. Table 4 shows maincharacteristics of the light-emitting element 2 and the light-emittingelement 3 at around 1000 cd/m².

TABLE 4 Current Current Power External Voltage Current densityChromaticity efficiency efficiency quantum (V) (mA) (mA/cm²) (x, y)(cd/A) (lm/W) efficiency (%) Light-emitting 4.6 0.083 2.1 (0.46, 0.44)47 32 22 element 2 Light-emitting 4.6 0.084 2.1 (0.44, 0.46) 52 36 22element 3

From the above, the light-emitting element 2 and the light-emittingelement 3 turned out to have excellent element characteristics. Inparticular, as can be seen from FIG. 20 and FIG. 21, the light-emittingelement 2 and the light-emitting element 3 each have extremely highemission efficiency and have a high external quantum efficiencyexceeding 20% at around a practical luminance (1000 cd/m²). Further,FIG. 22 shows that the light-emitting element 2 and the light-emittingelement 3 have favorable voltage-luminance characteristics, and aredriven at a low voltage. Thus, as is clear from FIG. 23, thelight-emitting element 2 and the light-emitting element 3 have favorablepower efficiency.

The above shows that the light-emitting element 2 and the light-emittingelement 3 each corresponding to one embodiment of the present inventionhave favorable element characteristics and provide lights from threekinds of emission center substances in a good balance.

Example 3

In this example, a method for fabricating a light-emitting element whichcorresponds to one embodiment of the present invention described in anyof Embodiments 1 to 3 and the characteristics thereof will be described.Structural formulae of organic compounds used in this example are shownbelow. In this example, a light-emitting element 4 in which thelight-emitting layer 113 includes the first light-emitting layer 113 a,the second light-emitting layer 113 b, and the third light-emittinglayer 113 c was fabricated.

Next, a method for fabricating the light-emitting element 4 in thisexample will be described below.

First, a film of indium oxide-tin oxide containing silicon oxide (ITSO)was formed over a glass substrate by a sputtering method, so that thefirst electrode 101 was formed. The thickness thereof was 110 nm and theelectrode area was 2 mm×2 mm. Here, the first electrode 101 functions asan anode of the light-emitting element.

Next, as pretreatment for forming the light-emitting element over thesubstrate, UV-ozone treatment was performed for 370 seconds afterwashing of a surface of the substrate with water and baking that wasperformed at 200° C. for one hour.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate was cooled down for about 30 minutes.

Then, the substrate over which the first electrode 101 was formed wasfixed to a substrate holder provided in the vacuum evaporation apparatusso that the surface on which the first electrode 101 was formed faceddownward. The pressure in the vacuum evaporation apparatus was reducedto about 10⁻⁴ Pa. After that, over the first electrode 101, DBT3P-II andmolybdenum(VI) oxide were deposited by co-evaporation by an evaporationmethod using resistance heating, so that the hole-injection layer 111was formed. The thickness of the hole-injection layer 111 was set to 40nm, and the weight ratio of DBT3P-II to molybdenum oxide was adjusted to4:2. Note that the co-evaporation method refers to an evaporation methodin which evaporation is carried out from a plurality of evaporationsources at the same time in one treatment chamber.

Next, a film of4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(PCBNBB)] which is represented by Structural Formula (xii) was formed toa thickness of 20 nm over the hole-injection layer 111 to form thehole-transport layer 112.

Over the hole-transport layer 112,2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II) represented by Structural Formula (xiii),PCBNBB, and [Ir(tppr)₂(dpm)] were deposited by co-evaporation to athickness of 20 nm with a weight ratio of 2mDBTBPDBq-II to PCBNBB and[Ir(tppr)₂(dpm)] being 0.5:0.5:0.05, so that the third light-emittinglayer 113 c was formed; then, 2mDBTBPDBq-II, PCBNBB, and[Ir(tBuppm)₂(acac)] were deposited by co-evaporation to a thickness of10 nm with a weight ratio of 2mDBTBPDBq-II to PCBNBB and[Ir(tBuppm)₂(acac)] being 0.5:0.5:0.05, so that the secondlight-emitting layer 113 b was formed; after that, 35DCzPPy, PCCP, and[Ir(mpptz-dmp)₃] were deposited by co-evaporation to a thickness of 30nm with a weight ratio of 35DCzPPy to PCCP and [Ir(mpptz-dmp)₃] being0.7:0.3:0.06, so that the first light-emitting layer 113 a was formed.

Then, the electron-transport layer 114 was formed over thelight-emitting layer 113 in such a way that a 10-nm-thick film of35DCzPPy was formed and a 20-nm-thick film of BPhen was formed.

After the formation of the electron-transport layer 114, lithiumfluoride (LiF) was deposited by evaporation to a thickness of 1 nm, sothat the electron-injection layer 115 was formed.

Lastly, aluminum was deposited by evaporation to a thickness of 200 nmto form the second electrode 102 functioning as a cathode. Thus, thelight-emitting element 4 in this example was fabricated.

Note that in all the above evaporation steps, evaporation was performedby a resistance-heating method.

Table 5 shows an element structure of the light-emitting element 4obtained in the above manner.

TABLE 5 Hole- Hole- Electron- First injection transport Light- Electron-injection Second electrode layer layer emitting layer transport layerlayer electrode Light-emitting ITSO DBT3P-II:MoOx PCBNBB * ** ***35DCzPPy Bphen LiF Al element 4 (110 nm) (4:2 40 nm) (20 nm) (10 nm) (20nm) (1 nm) (200 nm) * 2mDBTBPDBq-II:PCBNBB:[Ir(tppr)₂(dpm)](0.5:0.5:0.05 20 nm) ** 2mDBTBPDBq-II:PCBNBB:[Ir(tBuppm)₂(acac)](0.5:0.5:0.05 10 nm) *** 35DCzPPy:PCCP:[Ir(mpptz-dmp)₃] (0.7:0.3:0.06 30nm)

The light-emitting element 4 was sealed using a glass substrate in aglove box containing a nitrogen atmosphere so as not to be exposed tothe air (specifically, a sealing material was applied onto an outer edgeof the element and heat treatment was performed at 80° C. for 1 hour atthe time of sealing).

Element characteristics of the light-emitting element were measured.Note that the measurements were carried out at room temperature (in anatmosphere kept at 25° C.).

FIG. 24 shows an emission spectrum of the light-emitting element 4 whichwas obtained when a current of 0.1 mA was made to flow in thelight-emitting element 4. FIG. 24 indicates that the light-emittingelement 4 shows an emission spectrum including light with a wavelengthin the blue wavelength range which originates from [Ir(mpptz-dmp)₃],light with a wavelength in the green wavelength range which originatesfrom [Ir(tBuppm)₂(acac)], and light with a wavelength in the redwavelength range which originates from [Ir(tppr)₂(dpm)].

FIG. 25 shows luminance-current efficiency characteristics of thelight-emitting element 4; FIG. 26 shows luminance-external quantumefficiency characteristics thereof; FIG. 27 shows voltage-luminancecharacteristics thereof; and FIG. 28 shows luminance-power efficiencycharacteristics thereof. Table 6 shows main characteristics of thelight-emitting element 4 at around 1000 cd/m².

TABLE 6 Current Current Power External Voltage Current densityChromaticity efficiency efficiency quantum (V) (mA) (mA/cm²) (x, y)(cd/A) (lm/W) efficiency (%) Light-emitting 4.4 0.092 2.3 (0.53, 0.41)38 27 23 element 4

From the above, the light-emitting element 4 turned out to haveexcellent element characteristics. In particular, as can be seen fromFIG. 25 and FIG. 26, the light-emitting element 4 has extremely highemission efficiency and has a high external quantum efficiency exceeding20% at around a practical luminance (1000 cd/m²). Further, FIG. 27 showsthat the light-emitting element 4 has favorable voltage-luminancecharacteristics, and is driven at a low voltage. Thus, as is clear fromFIG. 28, the light-emitting element 4 has favorable power efficiency.

The above shows that the light-emitting element 4 corresponding to oneembodiment of the present invention has favorable elementcharacteristics and provides lights from three kinds of emission centersubstances in a good balance.

Further, FIG. 29 shows the results of a reliability test underconditions where the initial luminance was 3000 cd/m² and the currentdensity was constant. FIG. 29 shows a change in normalized luminancewith an initial luminance of 100%. The results show that a decrease inluminance over driving time of the light-emitting element 4 is small,and thus the light-emitting element 4 has favorable reliability.

Reference Example 1

The triplet excitation energies of 35DCzPPy, PCCP, 2mDBTPDBq-II,PCBA1BP, 2mDBTBPDBq-II, and PCBNBB used for the light-emitting elementsin the above examples were measured. Note that the triplet excitationenergies were measured in such a manner that phosphorescent emission ofeach substance was measured and a phosphorescence wavelength wasconverted into electron volt. In the measurement, each substance wasirradiated with excitation light with a wavelength of 325 nm and themeasurement temperature was 10 K. Note that in measuring an energylevel, calculation from an absorption wavelength is more accurate thancalculation from an emission wavelength. However, here, absorption ofthe triplet excitation energy was extremely low and difficult tomeasure; thus, the triplet excitation energy was measured by measuring apeak wavelength located on the shortest wavelength side in aphosphorescence spectrum. For that reason, a few errors may be includedin the measured values.

FIG. 30, FIG. 31, FIG. 32, FIG. 33, FIG. 34, and FIG. 35 show measuredphosphorescence. Table 7 shows the measurement results. As apparent fromthese results, the triplet excitation energies of 35DCzPPy and PCCP usedfor the first light-emitting layer are higher than those of2mDBTPDBq-II, PCBA1BP, 2mDBTBPDBq-II, and PCBNBB used for the secondlight-emitting layer or the third light-emitting layer.

TABLE 7 Triplet excitation energies of substances 35DCzPPy 2.74 eV PCCP2.64 eV 2mDBTPDBq-II 2.40 eV PCBA1BP 2.46 eV 2mDBTBPDBq-II 2.41 eVPCBNBB 2.21 eV

Reference Example 2

A synthetic example of an organometallic complex(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)(another name:bis[2-(6-tert-butyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ²O,O′)iridium(III))(abbreviation: [Ir(tBuppm)₂(acac)]), which is used in the aboveexamples, will be described. The structure of [Ir(tBuppm)₂(acac)] isshown below.

Step 1: Synthesis of 4-tert-butyl-6-phenylpyrimidine (abbreviation:HtBuppm)

First, 22.5 g of 4,4-dimethyl-1-phenylpentane-1,3-dione and 50 g offormamide were put into a recovery flask equipped with a reflux pipe,and the air in the flask was replaced with nitrogen. This reactioncontainer was heated, so that the reacted solution was refluxed for 5hours. After that, this solution was poured into an aqueous solution ofsodium hydroxide, and an organic layer was extracted withdichloromethane. The obtained organic layer was washed with water andsaturated saline, and dried with magnesium sulfate. The solution afterdrying was filtered. The solvent of this solution was distilled off, andthen the obtained residue was purified by silica gel columnchromatography using hexane and ethyl acetate as a developing solvent ina volume ratio of 10:1, so that a pyrimidine derivative HtBuppm(colorless oily substance, yield of 14%) was obtained. A synthesisscheme of Step 1 is shown below.

Step 2: Synthesis ofdi-μ-chloro-bis[bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)](abbreviation:[Ir(tBuppm)₂Cl]₂)

Next, 15 mL of 2-ethoxyethanol, 5 mL of water, 1.49 g of HtBuppmobtained in Step 1, and 1.04 g of iridium chloride hydrate (IrCl₃.H₂O)were put into a recovery flask equipped with a reflux pipe, and the airin the flask was replaced with argon. After that, irradiation withmicrowaves (2.45 GHz, 100 W) was performed for 1 hour to cause areaction. The solvent was distilled off, and then the obtained residuewas suction-filtered and washed with ethanol, so that a dinuclearcomplex [Ir(tBuppm)₂Cl]₂ (yellow green powder, yield of 73%) wasobtained. A synthesis scheme of Step 2 is shown below.

Step 3: Synthesis of(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)(abbreviation: [ft(tBuppm)₂(acac)])

Further, 40 mL of 2-ethoxyethanol, 1.61 g of the dinuclear complex[Ir(tBuppm)₂Cl]₂ obtained in Step 2, 0.36 g of acetylacetone, and 1.27 gof sodium carbonate were put into a recovery flask equipped with areflux pipe, and the air in the flask was replaced with argon. Afterthat, irradiation with microwaves (2.45 GHz, 120 W) was performed for 60minutes to cause a reaction. The solvent was distilled off, and theobtained residue was suction-filtered with ethanol and washed with waterand ethanol. This solid was dissolved in dichloromethane, and themixture was filtered through a filter aid in which Celite (produced byWako Pure Chemical Industries, Ltd., Catalog No. 531-16855), alumina,and Celite were stacked in this order. The solvent was distilled off,and the obtained solid was recrystallized with a mixed solvent ofdichloromethane and hexane, so that the objective substance was obtainedas yellow powder (yield of 68%). A synthesis scheme of Step 3 is shownbelow.

An analysis result by nuclear magnetic resonance spectrometry (¹H NMR)of the yellow powder obtained in Step 3 is described below. Theseresults revealed that the organometallic complex Ir(tBuppm)₂(acac) wasobtained.

¹H NMR. δ (CDCl₃): 1.50 (s, 18H), 1.79 (s, 6H), 5.26 (s, 1H), 6.33 (d,2H), 6.77 (t, 2H), 6.85 (t, 2H), 7.70 (d, 2H), 7.76 (s, 2H), 9.02 (s,2H).

Reference Example 3

In this reference example, a synthesis method oftris{2-[5-(2-methylphenyl)-4-(2,6-dimethylphenyl)-4H-1,2,4-triazol-3-yl-κN2]phenyl-κC}iridium(III)(abbreviation: [Ir(mpptz-dmp)₃]), which is used in the above example,will be described. A structure of [Ir(mpptz-dmp)₃] is shown below.

Step 1: Synthesis of N-benzoyl-N′-2-methylbenzoylhydrazide

First, 15.0 g (110.0 mmol) of benzoylhydrazine and 75 ml ofN-methyl-2-pyrrolidinone (NMP) were put into a 300-ml three-neck flaskand stirred while being cooled with ice. To this mixed solution, a mixedsolution of 17.0 g (110.0 mmol) of o-toluoyl chloride and 15 ml ofN-methyl-2-pyrrolidinone (NMP) was slowly added dropwise. After theaddition, the mixture was stirred at room temperature for 24 hours.After reaction for the predetermined time, this reacted solution wasslowly added to 500 ml of water, so that a white solid was precipitated.The precipitated solid was subjected to ultrasonic cleaning in whichwater and 1M hydrochloric acid were used alternately. Then, ultrasoniccleaning using hexane was performed, so that 19.5 g of a white solid ofN-benzoyl-N′-2-methylbenzoylhydrazide was obtained in a yield of 70%. Asynthesis scheme of Step 1 is shown below.

Step 2: Synthesis ofN-[1-chloro-1-(2-methylphenyl)methylidene]-N′-[1-chloro-(1-phenyl)methylidene]hydrazine

Next, 12.0 g (47.2 mmol) of N-benzoyl-N′-2-methylbenzoylhydrazideobtained in Step 1 and 200 ml of toluene were put into a 500-mlthree-neck flask. To this mixed solution, 19.4 g (94.4 mmol) ofphosphorus pentachloride was added and the mixture was heated andstirred at 120° C. for 6 hours. After reaction for the predeterminedtime, the reacted solution was slowly poured into 200 ml of water andthe mixture was stirred for 1 hour. After the stirring, an organic layerand an aqueous layer were separated, and the organic layer was washedwith water and a saturated aqueous solution of sodium hydrogencarbonate. After the washing, the organic layer was dried with anhydrousmagnesium sulfate. The magnesium sulfate was removed from this mixtureby gravity filtration, and the filtrate was concentrated; thus, 12.6 gof a brown liquid ofN-[1-chloro-1-(2-methylphenyl)methylidene]-N′-[1-chloro-(1-phenyl)methylidene]hydrazine was obtained in a yield of 92%. A synthesis scheme of Step 2 isshown below.

Step 3: Synthesis of3-(2-methylphenyl)-4-(2,6-dimethylphenyl)-5-phenyl-4H-1,2,4-triazole(abbreviation: Hmpptz-dmp)

First, 12.6 g (43.3 mmol) ofN-[1-chloro-1-(2-methylphenyl)methylidene]-N′-[1-chloro-(1-phenyl)methylidene]hydrazine obtained in Step 2, 15.7 g (134.5 mmol) of 2,6-dimethylaniline,and 100 ml of N,N-dimethylaniline were put into a 500-ml recovery flaskand heated and stirred at 120° C. for 20 hours. After reaction for thepredetermined time, this reacted solution was slowly added to 200 ml of1N hydrochloric acid. Dichloromethane was added to this solution and anobjective substance was extracted to an organic layer. The obtainedorganic layer was washed with water and an aqueous solution of sodiumhydrogen carbonate, and was dried with magnesium sulfate. The magnesiumsulfate was removed by gravity filtration, and the obtained filtrate wasconcentrated to give a black liquid. This liquid was purified by silicagel column chromatography. A mixed solvent of ethyl acetate and hexanein a ratio of 1:5 was used as a developing solvent. The obtainedfraction was concentrated to give a white solid. This solid wasrecrystallized with ethyl acetate to give 4.5 g of a white solid ofHmpptz-dmp in a yield of 31%. A synthesis scheme of Step 3 is shownbelow.

Step 4: Synthesis of [Ir(mpptz-dmp)₃]

Then, 2.5 g (7.4 mmol) of Hmpptz-dmp, which was the ligand obtained inStep 3, and 0.7 g (1.5 mmol) of tris(acetylacetonato)iridium(III) wereput into a container for high-temperature heating, and degasificationwas carried out. The mixture in the reaction container was heated andstirred at 250° C. for 48 hours under Ar flow. After reaction for thepredetermined time, the obtained solid was washed with dichloromethane,and an insoluble green solid was obtained by suction filtration. Thissolid was dissolved in toluene and filtered through a stack of aluminaand Celite. The obtained fraction was concentrated to give a greensolid. This solid was recrystallized with toluene, so that 0.8 g of agreen powder was obtained in a yield of 45%. A synthesis scheme of Step4 is shown below.

An analysis result by nuclear magnetic resonance (¹H-NMR) spectroscopyof the green powder obtained in Step 4 is described below. The resultrevealed that [Ir(mpptz-dmp)₃] was obtained by the synthesis method.

¹H-NMR. δ (toluene-d8): 1.82 (s, 9H), 1.90 (s, 9H), 2.64 (s, 9H),6.56-6.62 (m, 9H), 6.67-6.75 (m, 9H), 6.82-6.88 (m, 3H), 6.91-6.97 (t,3H), 7.00-7.12 (m, 6H), 7.63-7.67 (d, 3H).

This application is based on Japanese Patent Application serial no.2012-177795 filed with Japan Patent Office on Aug. 10, 2012, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A light-emitting device comprising: an anode; afirst light-emitting layer over the anode, the first light-emittinglayer comprising a first phosphorescent compound and a first hostmaterial; a second light-emitting layer between the anode and the firstlight-emitting layer and in contact with the first light-emitting layer,the second light-emitting layer comprising a second phosphorescentcompound, a first electron-transport compound, and a firsthole-transport compound; and a cathode over the first light-emittinglayer, wherein an emission wavelength of the second phosphorescentcompound is longer than an emission wavelength of the firstphosphorescent compound, wherein a triplet excitation energy of thefirst host material is higher than or equal to a triplet excitationenergy of the first electron-transport compound or the firsthole-transport compound, wherein the first electron-transport compoundand the first hole-transport compound form an exciplex, and wherein adifference in energy between an emission peak wavelength of the exciplexand a peak wavelength of an absorption band of the second phosphorescentcompound which originates from a triplet metal to ligand charge transfertransition, is 0.2 eV or less.
 2. The light-emitting device according toclaim 1, wherein the first host material has a hole-transport property.3. The light-emitting device according to claim 1, wherein the firstlight-emitting layer further comprises a second host material, whereinthe second host material has an electron-transport property, and whereina triplet excitation energy of the second host material is higher thanor equal to the triplet excitation energy of the firstelectron-transport compound or the first hole-transport compound.
 4. Thelight-emitting device according to claim 3, wherein the first hostmaterial and the second host material form an exciplex.
 5. A lightingdevice comprising the light-emitting device according to claim
 1. 6. Anelectronic device comprising the light-emitting device according toclaim
 1. 7. A light-emitting device comprising: an anode; a firstlight-emitting layer over the anode, the first light-emitting layercomprising a first phosphorescent compound and a first host material; asecond light-emitting layer between the first light-emitting layer andthe anode and in contact with the first light-emitting layer, the secondlight-emitting layer comprising a second phosphorescent compound, afirst electron-transport compound, and a first hole-transport compound;a third light-emitting layer between the second light-emitting layer andthe anode and in contact with the second light-emitting layer, the thirdlight-emitting layer comprising a third phosphorescent compound, asecond electron-transport compound, and a second hole-transportcompound; and a cathode over the first light-emitting layer, wherein anemission wavelength of the second phosphorescent compound is longer thanan emission wavelength of the first phosphorescent compound, wherein anemission wavelength of the third phosphorescent compound is longer thanthe emission wavelength of the second phosphorescent compound, wherein atriplet excitation energy of the first host material is higher than orequal to a triplet excitation energy of the first electron-transportcompound or the first hole-transport compound, wherein the firstelectron-transport compound and the first hole-transport compound forman exciplex, and wherein a difference in energy between an emission peakwavelength of the exciplex and a peak wavelength of an absorption bandof the second phosphorescent compound, which originates from a tripletmetal to ligand charge transfer transition is 0.2 eV or less.
 8. Thelight-emitting device according to claim 7, wherein the tripletexcitation energies of the first electron-transport compound and thefirst hole-transport compound are higher than a triplet excitationenergy of the second electron-transport compound or the secondhole-transport compound.
 9. The light-emitting device according to claim7, wherein the first host material has an electron-transport property.10. The light-emitting device according to claim 7, wherein the firstlight-emitting layer further comprises a second host material, whereinthe second host material has a hole-transport property, and wherein atriplet excitation energy of the second host material is higher than orequal to the triplet excitation energy of the first electron-transportcompound or the first hole-transport compound.
 11. The light-emittingdevice according to claim 10, wherein the first host material and thesecond host material form an exciplex.
 12. The light-emitting deviceaccording to claim 7, wherein the first electron-transport compound andthe second electron-transport compound are the same material.
 13. Thelight-emitting device according to claim 7, wherein the firsthole-transport compound and the second hole-transport compound are thesame material.
 14. The light-emitting device according to claim 7,wherein the first electron-transport compound and the secondelectron-transport compound are the same material, and wherein the firsthole-transport compound and the second hole-transport compound are thesame material.
 15. A lighting device comprising the light-emittingdevice according to claim
 7. 16. An electronic device comprising thelight-emitting device according to claim 7.